View source for List of counties in Illinois

{{Short description|None}}
{{Use American English|date=June 2025}}
{{Use mdy dates|date=April 2024}}
{{Infobox subdivision type
| name = Counties of Illinois
| map = {{Illinois County Labelled Map}}
| territory = [[State of Illinois]]
| start_date = 
| current_number = 102
| number_date = 
| population_range = 3,550 ([[Hardin County, Illinois|Hardin]]) – 5,182,617 ([[Cook County, Illinois|Cook]])
| area_range = {{Convert|172|sqmi}} ([[Putnam County, Illinois|Putnam]]) – {{Convert|1186|sqmi}} ([[McLean County, Illinois|McLean]])
| government = [[County government]]
| subdivision = [[List of precincts in Illinois|255 Precincts]] <br/> [[List of Illinois townships|1,426 Townships]]
}}

There are 102 [[County (United States)|counties]] in [[Illinois]]. The most populous of these is [[Cook County, Illinois|Cook County]], the second-most populous county in the United States and the home of [[Chicago]], while the least populous is [[Hardin County, Illinois|Hardin County]]. The largest by land area is [[McLean County, Illinois|McLean County]], while the smallest is [[Putnam County, Illinois|Putnam County]]. Illinois's [[FIPS state code]] is 17 and its postal abbreviation is IL.

What is now Illinois was claimed as part of [[Illinois County, Virginia]], between 1778 and 1782. Modern-day county formation dates to 1790 when the area was part of the [[Northwest Territory]]; two counties — St. Clair and Knox — were created at that time. Knox would later become [[Knox County, Indiana|a county in Indiana]] and is unrelated to the current Knox County in Illinois, while St. Clair would become [[St. Clair County, Illinois|the oldest county in Illinois]]. Successive territorial governments created 15 counties by the time Illinois [[admitted to the Union|achieved statehood]] in 1818. The last county created in the state, [[Ford County, Illinois|Ford County]], was created by the Illinois government in 1859.

Most counties in Illinois were named after early American leaders, especially of the [[American Revolutionary War]], as well as soldiers from the [[Battle of Tippecanoe]] and the [[War of 1812]]. Some are named after natural features or counties in other states. Some are named for early Illinois leaders. Two counties are named for Native American tribes, and one bears the name of a plant used as a food source by Native Americans.

While it does have [[Lincoln, Illinois|a Lincoln city]], Illinois does not have a county named after [[Abraham Lincoln]]; it does, however, have a [[Douglas County, Illinois|Douglas County]] (founded 1859) named after his political rival [[Stephen A. Douglas]]. It also has [[Calhoun County, Illinois|Calhoun County]] (founded 1825), named after [[John C. Calhoun]], outspoken for his pro-slavery and pro-southern views in the years preceding the [[American Civil War]]. Several of the counties are named after Southerners, reflecting the fact that Illinois was [[Illinois County, Virginia|for a short time]] part of [[Colony of Virginia|Virginia]], and settled in its early years by many Southerners. No counties are named after heroes of the [[American Civil War|Civil War]], mainly because the counties were all named before that war. The state does have a [[Lee County, Illinois|Lee County]] (founded 1839) named after the American Revolution's [[Henry Lee III]], the father of [[Robert E. Lee]] from the Civil War, who at one time served in Illinois. Illinois also has two counties named after the same person, the prominent early 19th century New York governor and presidential candidate [[DeWitt Clinton]] (DeWitt County, and Clinton County).

Information on the [[FIPS county code]], [[county seat]], year of establishment, origin, [[etymology]], population, area and map of each county is included in the table below.

==Counties==
''Note: the links in the [[FIPS county code|FIPS County Code]] column are to the [[United States Census Bureau]] page for that county.''
{{Clear}}
{{Countytabletop
| region_width      = 
| fips_ref          = <ref name="FIPS">{{cite web |url=https://www2.census.gov/programs-surveys/popest/geographies/2019/all-geocodes-v2019.xlsx |title=U.S. Census Bureau 2019 FIPS Codes |work=census.gov |access-date=January 8, 2021 }}</ref>
| region_seat_title = County seat
| region_seat_width =
| region_seat_ref = <ref name="NACO">{{cite web|url=https://ce.naco.org/?find=true|title=County Explorer|publisher=[[National Association of Counties]]|access-date=January 8, 2021}}</ref>
| data2_title       = {{abbr|Est.|Established}}
| data2_width       =
| data2_ref         = <ref name="NACO">{{cite web|url=https://ce.naco.org/?find=true|title=County Explorer|publisher=[[National Association of Counties]]|access-date=January 8, 2021}}</ref>
| data3_title       = Origin
| data3_width       =
| data3_ref         = 
| data4_title       = Etymology
| data4_width       =
| data4_ref         = <ref>{{cite web |url=http://genealogytrails.com/ill/cobiosad.htm|title=Illinois County Biographies |access-date=September 21, 2008 |work=Genealogy Trails.com|publisher=Genealogy Trails|year=2000 }}</ref><ref>{{cite web |url=http://genealogytrails.com/ill/ilcohx.htm|title=The Origin of Illinois County Names|access-date=September 21, 2008 |work=Genealogy Trails.com|publisher=Genealogy Trails |year=2000 }}</ref>
| data4_unsortable  = yes
| population_ref    = <ref>{{cite web |url=https://www.census.gov/quickfacts/fact/table/IL/PST045219 |title=U.S. Census Bureau QuickFacts: Illinois |access-date=March 15, 2025 }}</ref>
| area_ref          = <ref name="NACO"/>
}}
{{Countyrow|N=17|Num=001| Name=Adams| Seat=Quincy| Data2=1825| Data3=Pike County |Data4=[[John Quincy Adams]] (1767–1848), sixth [[President of the United States]] | Population=64109| Area=855| Size=60px}}
{{Countyrow|N=17|Num=003| Name=Alexander| Seat=Cairo| Data2=1819| Data3=Union County |Data4=[[William Melville Alexander (Illinois politician)|William M. Alexander]], settler and state representative in the [[Illinois General Assembly]] | Population=4594| Area=236| Size=60px}}
{{Countyrow|N=17|Num=005| Name=Bond| Seat=Greenville| Data2=1817| Data3=Crawford County, Edwards County, and Madison County |Data4=[[Shadrach Bond]] (1773–1832), first [[Governor of Illinois]] | Population=16576| Area=380| Size=60px}}
{{Countyrow|N=17|Num=007| Name=Boone| Seat=Belvidere| Data2=1837| Data3=Winnebago County |Data4=[[Daniel Boone]] (1734–1820), trailblazer of the [[Wilderness Road]] in Kentucky | Population=53313| Area=280| Size=60px}}
{{Countyrow|N=17|Num=009| Name=Brown| Seat=Mount Sterling| Data2=1839| Data3=Schuyler County |Data4=[[Jacob Brown (general)|Jacob Brown]] (1775–1828), successful [[War of 1812]] army officer responsible for Great Lakes defenses | Population=6307| Area=305| Size=60px}}
{{Countyrow|N=17|Num=011| Name=Bureau| Seat=Princeton| Data2=1837| Data3=Putnam County |Data4=Pierre de Bureo, Frenchman, [[Fur trade#North American fur trade|North American fur trader]]| Population=32486| Area=869| Size=60px}}
{{Countyrow|N=17|Num=013| Name=Calhoun| Seat=Hardin| Data2=1825| Data3=Pike County |Data4=[[John C. Calhoun]] (1782–1850), [[South Carolina]] senator and seventh [[Vice President of the United States]] | Population=4224| Area=253| Size=60px}}
{{Countyrow|N=17|Num=015| Name=Carroll| Seat=Mount Carroll| Data2=1839| Data3=Jo Daviess |Data4=[[Charles Carroll of Carrollton]] (1737–1832), signed the [[United States Declaration of Independence|Declaration of Independence]] on behalf of [[Maryland]] | Population=15444| Area=445| Size=60px}}
{{Countyrow|N=17|Num=017| Name=Cass| Seat=Virginia| Data2=1837| Data3=Morgan County |Data4=[[Lewis Cass]] (1782–1866), second governor of [[Michigan Territory]], fourteenth [[United States Secretary of War]] | Population=12713| Area=375| Size=60px}}
{{Countyrow|N=17|Num=019| Name=Champaign| Seat=Urbana| Data2=1833| Data3=Vermilion County |Data4=[[Champaign County, Ohio]], which took its name from the [[French language|French]] for "open level country" | Population=212374| Area=996| Size=60px}}
{{Countyrow|N=17|Num=021| Name=Christian| Seat=Taylorville| Data2=1839| Data3=Sangamon County |Data4= [[Christian County, Kentucky]], which was named after [[William Christian (Virginia politician)|William Christian]] | Population=33247| Area=709| Size=60px}}
{{Countyrow|N=17|Num=023| Name=Clark| Seat=Marshall| Data2=1819| Data3=Crawford County |Data4=[[George Rogers Clark]] (1752–1818), highest-ranking officer in the [[Northwest Territory]] during the [[American Revolution]] | Population=15076| Area=501| Size=60px}}
{{Countyrow|N=17|Num=025| Name=Clay| Seat=Louisville| Data2=1824| Data3=Wayne, Lawrence, Fayette, and Crawford County |Data4=[[Henry Clay]] (1777–1852), Kentucky legislator who negotiated the [[Missouri Compromise]] | Population=12821| Area=468| Size=60px}}
{{Countyrow|N=17|Num=027| Name=Clinton| Seat=Carlyle| Data2=1824| Data3=Washington, Bond, and Fayette County |Data4=[[DeWitt Clinton]] (1769–1828), [[List of Governors of New York|Governor of New York]], responsible for the construction of the [[Erie Canal]] | Population=37087| Area=474| Size=60px}}
{{Countyrow|N=17|Num=029| Name=Coles| Seat=Charleston| Data2=1830| Data3=Clark and Edgar County |Data4=[[Edward Coles]] (1786–1868), second [[Governor of Illinois]], responsible for the abolition of slavery in Illinois | Population=46423| Area=508| Size=60px}}
{{Countyrow|N=17|Num=031| Name=Cook| Seat=Chicago| Data2=1831| Data3=Putnam County |Data4=[[Daniel Pope Cook]] (1794–1827), politician and first [[Illinois Attorney General|Attorney General of Illinois]] | Population=5182617| Area=944| Size=60px}}
{{Countyrow|N=17|Num=033| Name=Crawford| Seat=Robinson| Data2=1816| Data3=Edwards County |Data4=[[William H. Crawford]] (1772–1834), ninth [[United States Secretary of War]], seventh [[United States Secretary of the Treasury|Secretary of the Treasury]] | Population=18388| Area=443| Size=60px}}
{{Countyrow|N=17|Num=035| Name=Cumberland| Seat=Toledo| Data2=1843| Data3=Coles County |Data4=''Disputed'': [[National Road|Cumberland Road]], which entered the county; [[Cumberland, Maryland]]; or [[Cumberland River]] in Kentucky | Population=10305| Area=345| Size=60px}}
{{Countyrow|N=17|Num=037| Name=DeKalb| Seat=Sycamore| Data2=1837| Data3=Kane County |Data4=[[Johann de Kalb]] (1721–1780), German officer in the [[Continental Army]] who fought alongside [[Gilbert du Motier, marquis de Lafayette]] | Population=101335| Area=631| Size=60px}}
{{Countyrow|N=17|Num=039| Name=DeWitt| Seat=Clinton| Data2=1839| Data3=Macon and McLean County |Data4=[[DeWitt Clinton]] (1769–1828), [[List of Governors of New York|Governor of New York]], responsible for the construction of the [[Erie Canal]] | Population=15319| Area=397| Size=60px|Map=Map of Illinois highlighting DeWitt County.svg}}
{{Countyrow|N=17|Num=041| Name=Douglas| Seat=Tuscola| Data2=1859| Data3=Coles County |Data4=[[Stephen A. Douglas]] (1813–1861), prominent Illinois Democrat who [[Lincoln–Douglas debates|engaged in debates]] with [[Abraham Lincoln]] | Population=19824| Area=416| Size=60px}}
{{Countyrow|N=17|Num=043| Name=DuPage| Seat=Wheaton| Data2=1839| Data3=Cook County |Data4=[[DuPage River]]
| Population=937142| Area=327| Size=60px}}
{{Countyrow|N=17|Num=045| Name=Edgar| Seat=Paris| Data2=1823| Data3=Clark County |Data4=[[John Edgar (politician)|John Edgar]] ({{abbr|c.|circa}} 1750–1832), Illinois delegate to the [[Northwest Territory]] legislature; at the time, wealthiest man in Illinois
 | Population=16398| Area=623| Size=60px}}
{{Countyrow|N=17|Num=047| Name=Edwards| Seat=Albion| Data2=1814| Data3=Gallatin County and Madison County |Data4=[[Ninian Edwards]] (1775–1833), third [[Governor of Illinois|Governor of the State of Illinois]] and only governor of the [[Illinois Territory]]
 | Population=5984| Area=222| Size=60px}}
{{Countyrow|N=17|Num=049| Name=Effingham| Seat=Effingham| Data2=1831| Data3=Fayette and Crawford County |Data4= [[Thomas Howard, 3rd Earl of Effingham]], military officer who resigned from the [[British Army]] to avoid fighting the American colonies
 | Population=34602| Area=478| Size=60px}}
{{Countyrow|N=17|Num=051| Name=Fayette| Seat=Vandalia| Data2=1821| Data3=Bond, Wayne, Clark, Jefferson, and Crawford County |Data4=[[Gilbert du Motier, marquis de Lafayette]] (1757–1834), French military officer who was a key factor in the [[American Revolution|American]] and [[French Revolution]]s.
 | Population=21157| Area=716| Size=60px}}
{{Countyrow|N=17|Num=053| Name=Ford| Seat=Paxton| Data2=1859| Data3=Vermilion County |Data4=[[Thomas Ford (politician)|Thomas Ford]] (1800–1850), eighth [[Governor of Illinois]]; served during the [[History of Nauvoo, Illinois|Illinois Mormon War]]
 | Population=13451| Area=485| Size=60px}}
{{Countyrow|N=17|Num=055| Name=Franklin| Seat=Benton| Data2=1818| Data3=White County and Gallatin County |Data4=[[Benjamin Franklin]] (1706–1790), prolific writer, inventor, and politician; key factor in the [[American Revolution]]
 | Population=37033| Area=408| Size=60px}}
{{Countyrow|N=17|Num=057| Name=Fulton| Seat=Lewistown| Data2=1823| Data3=Pike County |Data4=[[Robert Fulton]] (1765–1815), inventor of the [[steamboat]]
 | Population=32510| Area=865| Size=60px}}
{{Countyrow|N=17|Num=059| Name=Gallatin| Seat=Shawneetown| Data2=1812| Data3=Randolph County |Data4=[[Albert Gallatin]] (1761–1849), fourth and longest-serving [[United States Secretary of the Treasury]]
 | Population=4690| Area=322| Size=60px}}
{{Countyrow|N=17|Num=061| Name=Greene| Seat=Carrollton| Data2=1821| Data3=Madison County |Data4=[[Nathanael Greene]] (1742–1786), major general in the [[Continental Army]]
 | Population=11417| Area=543| Size=60px}}
{{Countyrow|N=17|Num=063| Name=Grundy| Seat=Morris| Data2=1841| Data3=LaSalle County |Data4=[[Felix Grundy]] (1777–1840), [[Tennessee|Tennessean]] senator who served as the thirteenth [[United States Attorney General]]
 | Population=53835| Area=418| Size=60px}}
{{Countyrow|N=17|Num=065| Name=Hamilton| Seat=McLeansboro| Data2=1821| Data3=White County |Data4=[[Alexander Hamilton]] (1755–1804), first [[United States Secretary of the Treasury]] | Population=7818| Area=434| Size=60px}}
{{Countyrow|N=17|Num=067| Name=Hancock| Seat=Carthage| Data2=1825| Data3=Adams County |Data4=[[John Hancock]] (1737–1793), first governor of the [[Massachusetts|Massachusetts colony]] and president of the [[Second Continental Congress]] | Population=17008| Area=793| Size=60px}}
{{Countyrow|N=17|Num=069| Name=Hardin| Seat=Elizabethtown| Data2=1839| Data3=Pope County |Data4=[[Hardin County, Kentucky]], which was named after [[John Hardin]]
| Population=3550| Area=177| Size=60px}}
{{Countyrow|N=17|Num=071| Name=Henderson| Seat=Oquawka| Data2=1841| Data3=Warren County |Data4=[[Henderson County, Kentucky]], which was named after [[Richard Henderson (jurist)|Richard Henderson]] | Population=6048| Area=378| Size=60px}}
{{Countyrow|N=17|Num=073| Name=Henry| Seat=Cambridge| Data2=1825| Data3=Fulton County |Data4=[[Patrick Henry]] (1736–1799), [[American Revolutionary War]] figure; first and sixth [[Governor of Virginia]] | Population=48177| Area=822| Size=60px}}
{{Countyrow|N=17|Num=075| Name=Iroquois| Seat=Watseka| Data2=1833| Data3=Vermilion County |Data4=[[Iroquois]] Native Americans | Population=26067| Area=1117| Size=60px}}
{{Countyrow|N=17|Num=077| Name=Jackson| Seat=Murphysboro| Data2=1816| Data3=Randolph County and Johnson County |Data4=[[Andrew Jackson]] (1767–1845), seventh [[President of the United States]], United States Senator from [[Tennessee]], and general in the [[War of 1812]] | Population=53199| Area=584| Size=60px}}
{{Countyrow|N=17|Num=079| Name=Jasper| Seat=Newton| Data2=1831| Data3=Clay and Crawford County |Data4=Sgt. [[William Jasper]] ({{abbr|c.|circa}} 1750–1779), [[American Revolutionary War]] soldier popularized by [[Parson Weems]] | Population=9092| Area=494| Size=60px}}
{{Countyrow|N=17|Num=081| Name=Jefferson| Seat=Mount Vernon| Data2=1819| Data3=Edwards and White County |Data4=[[Thomas Jefferson]] (1743–1826), third [[President of the United States]], second [[Vice President of the United States]], [[Governor of Virginia]], and one of the foremost [[Founding Fathers of the United States]] | Population=36107| Area=571| Size=60px}}
{{Countyrow|N=17|Num=083| Name=Jersey| Seat=Jerseyville| Data2=1839| Data3=Greene County |Data4=State of [[New Jersey]], from which many early settlers hailed | Population=21150| Area=369| Size=60px}}
{{Countyrow|N=17|Num=085| Name=Jo Daviess| Seat=Galena| Data2=1827| Data3=Henry, Mercer, and Putnam County |Data4=[[Joseph Hamilton Daveiss]] (1774–1811), commander of the Indiana Dragoons at the [[Battle of Tippecanoe]] | Population=21528| Area=600| Size=60px}}
{{Countyrow|N=17|Num=087| Name=Johnson| Seat=Vienna| Data2=1812| Data3=Randolph County |Data4=[[Richard Mentor Johnson]] ({{abbr|c.|circa}} 1780–1850), ninth [[Vice President of the United States]] and United States Senator from [[Kentucky]] | Population=13320| Area=343| Size=60px}}
{{Countyrow|N=17|Num=089| Name=Kane| Seat=Geneva| Data2=1836| Data3=LaSalle County |Data4=[[Elias Kane]] (1794–1835), United States Senator from [[Illinois]] | Population=520997| Area=519| Size=60px}}
{{Countyrow|N=17|Num=091| Name=Kankakee| Seat=Kankakee| Data2=1853| Data3=Iroquois and Will County |Data4=[[Kankakee River]] | Population=106410| Area=676| Size=60px}}
{{Countyrow|N=17|Num=093| Name=Kendall| Seat=Yorkville| Data2=1841| Data3=LaSalle and Kane County |Data4=[[Amos Kendall]] (1789–1869), [[United States Postmaster General]] under Presidents [[Andrew Jackson]] and [[Martin Van Buren]] | Population=143171| Area=320| Size=60px}}
{{Countyrow|N=17|Num=095| Name=Knox| Seat=Galesburg| Data2=1825| Data3=Fulton County |Data4=Gen. [[Henry Knox]] (1750–1806), [[American Revolutionary War]] general and first [[United States Secretary of War]] | Population=48716| Area=716| Size=60px}}
{{Countyrow|N=17|Num=097| Name=Lake| Seat=Waukegan| Data2=1839| Data3=McHenry County |Data4=[[Lake Michigan]] | Population=718604| Area=443| Size=60px}}
{{Countyrow|N=17|Num=099| Name=LaSalle| Seat=Ottawa| Data2=1831| Data3=Putnam and Tazewell County |Data4=[[René-Robert Cavelier, Sieur de La Salle|Sieur de la Salle]] (1643–1687), French explorer of the [[Great Lakes]] | Population=108390| Area=1135| Size=60px}}
{{Countyrow|N=17|Num=101| Name=Lawrence| Seat=Lawrenceville| Data2=1821| Data3=Crawford and Edwards County |Data4=Capt. [[James Lawrence]] (1781–1813), commander of the {{USS|Chesapeake|1799|6}} in the [[War of 1812]].  Famous for his command "Don't give up the ship!" | Population=14801| Area=372| Size=60px}}
{{Countyrow|N=17|Num=103| Name=Lee| Seat=Dixon| Data2=1839| Data3=Ogle County |Data4="Light Horse" [[Henry Lee III]] (1756–1818), [[American Revolutionary War]] officer and ninth [[Governor of Virginia]] | Population=33544| Area=724| Size=60px}}
{{Countyrow|N=17|Num=105| Name=Livingston| Seat=Pontiac| Data2=1837| Data3=LaSalle and McLean County |Data4=[[Edward Livingston]] (1764–1836), prominent jurist, Congressman from [[New York (state)|New York]] and [[Louisiana]], and [[U.S. Secretary of State]] from 1831-33 | Population=35375| Area=1043| Size=60px}}
{{Countyrow|N=17|Num=107| Name=Logan| Seat=Lincoln| Data2=1839| Data3=Sangamon County |Data4=John Logan, country doctor and early settler, and the father of [[John A. Logan]] | Population=27518| Area=618| Size=60px}}
{{Countyrow|N=17|Num=115| Name=Macon| Seat=Decatur| Data2=1829| Data3=Shelby County |Data4=[[Nathaniel Macon]] (1758–1837), sixth [[Speaker of the United States House of Representatives]] and [[United States Senator]] from [[North Carolina]] | Population=100737| Area=580| Size=60px}}
{{Countyrow|N=17|Num=117| Name=Macoupin| Seat=Carlinville| Data2=1829| Data3=Greene County |Data4=Native American word ''macoupin'', meaning "[[American lotus]]" | Population=43895| Area=862| Size=60px}}
{{Countyrow|N=17|Num=119| Name=Madison| Seat=Edwardsville| Data2=1812| Data3=St. Clair County and Randolph County |Data4=[[James Madison]] (1751–1836), fourth [[President of the United States]] and principal author of the [[Constitution of the United States]] | Population=263017| Area=716| Size=60px}}
{{Countyrow|N=17|Num=121| Name=Marion| Seat=Salem| Data2=1823| Data3=Fayette and Jefferson County |Data4=[[Francis Marion]] ({{abbr|c.|circa}} 1732–1795), general in the [[American Revolutionary War]] known as "The Swamp Fox" | Population=36437| Area=572| Size=60px}}
{{Countyrow|N=17|Num=123| Name=Marshall| Seat=Lacon| Data2=1839| Data3=Putnam County |Data4=John Marshall (1755–1835), fourth and longest-serving [[Chief Justice of the United States]], wrote opinion in ''[[Marbury v. Madison]]'' establishing the principle of [[judicial review]] | Population=11552| Area=386| Size=60px}}
{{Countyrow|N=17|Num=125| Name=Mason| Seat=Havana| Data2=1841| Data3=Tazewell and Menard County |Data4=Named after [[Mason County, Kentucky]], itself named after [[George Mason]] | Population=12543| Area=539| Size=60px}}
{{Countyrow|N=17|Num=127| Name=Massac| Seat=Metropolis| Data2=1843| Data3=Pope and Johnson County |Data4=[[Fort Massac]], a colonial-era fort on the [[Ohio River]] | Population=13627| Area=237| Size=60px}}
{{Countyrow|N=17|Num=109| Name=McDonough| Seat=Macomb| Data2=1826| Data3=Schuyler County |Data4=Commodore [[Thomas Macdonough]] (1783–1825), commander of American naval forces at the [[Battle of Plattsburgh]] | Population=26662| Area=589| Size=60px}}
{{Countyrow|N=17|Num=111| Name=McHenry| Seat=Woodstock| Data2=1836| Data3=Cook and LaSalle County |Data4=Major [[William McHenry]] ({{abbr|c.|circa}} 1771–1835), officer during several campaigns against [[Indigenous peoples of the Americas|Native Americans]] and was a member of the Illinois legislature | Population=315959| Area=603| Size=60px}}
{{Countyrow|N=17|Num=113| Name=McLean| Seat=Bloomington| Data2=1830| Data3=Tazewell County |Data4=[[John McLean (Illinois politician)|John McLean]] (1791–1830), [[United States Representative]] and [[United States Senator]] from Illinois (the latter from 1824–25 and 1829–30)| Population=172069| Area=1183| Size=60px}}
{{Countyrow|N=17|Num=129| Name=Menard| Seat=Petersburg| Data2=1839| Data3=Sangamon County |Data4=[[Pierre Menard]] (1766–1844), prominent early settler and first [[Lieutenant Governor of Illinois]] | Population=11963| Area=314| Size=60px}}
{{Countyrow|N=17|Num=131| Name=Mercer| Seat=Aledo| Data2=1825| Data3=Schuyler County |Data4=[[Hugh Mercer]] (1726–1777), British officer in the [[Seven Years' War]] and general in the [[Continental Army]] during the [[American Revolutionary War]] | Population=15292| Area=561| Size=60px}}
{{Countyrow|N=17|Num=133| Name=Monroe| Seat=Waterloo| Data2=1816| Data3=Randolph County and St. Clair County |Data4=[[James Monroe]] (1758–1831), seventh [[United States Secretary of State]], eighth [[United States Secretary of War]], [[Governor of Virginia]], and fifth [[President of the United States]] | Population=34969| Area=385| Size=60px}}
{{Countyrow|N=17|Num=135| Name=Montgomery| Seat=Hillsboro| Data2=1821| Data3=Bond and Madison County |Data4=Gen. [[Richard Montgomery]] (1738–1775), brigadier-general in the [[Continental Army]] who led the unsuccessful [[Invasion of Canada (1775)|invasion of Canada]] | Population=27615| Area=703| Size=60px}}
{{Countyrow|N=17|Num=137| Name=Morgan| Seat=Jacksonville| Data2=1823| Data3=Sangamon County |Data4=Gen. [[Daniel Morgan]] (1736–1802), successful tactician in the [[American Revolutionary War]] and later [[United States Representative]] from [[Virginia]]| Population=32618| Area=568| Size=60px}}
{{Countyrow|N=17|Num=139| Name=Moultrie| Seat=Sullivan| Data2=1843| Data3=Shelby and Macon County |Data4=Gen. [[William Moultrie]] (1730–1805), [[American Revolutionary War]] general and [[Governor of South Carolina]] | Population=14365| Area=335| Size=60px}}
{{Countyrow|N=17|Num=141| Name=Ogle| Seat=Oregon| Data2=1836| Data3=Jo Daviess |Data4=[[Joseph Ogle]] (1737–1821), early settler in southwest Illinois, who helped found the first [[Methodism|Methodist]] church in Illinois | Population=51345| Area=758| Size=60px}}
{{Countyrow|N=17|Num=143| Name=Peoria| Seat=Peoria| Data2=1825| Data3=Fulton County |Data4=The [[Peoria tribe|Peoria]] Native American tribe | Population=179630| Area=618| Size=60px}}
{{Countyrow|N=17|Num=145| Name=Perry| Seat=Pinckneyville| Data2=1827| Data3=Randolph and Jackson County |Data4=Cmdre. [[Oliver Hazard Perry]] (1785–1819), American naval officer who led the victorious American forces at the [[Battle of Lake Erie]] | Population=20272| Area=441| Size=60px}}
{{Countyrow|N=17|Num=147| Name=Piatt| Seat=Monticello| Data2=1841| Data3=DeWitt and Macon County |Data4=James A. Piatt, the patriarch of a prominent settler family in the early history of the county | Population=16628| Area=439| Size=60px}}
{{Countyrow|N=17|Num=149| Name=Pike| Seat=Pittsfield| Data2=1821| Data3=Madison, Bond, and Clark County |Data4=[[Zebulon Pike]] (1779–1813), early explorer of the [[American Southwest]], namesake of [[Pikes Peak]] | Population=14258| Area=831| Size=60px}}
{{Countyrow|N=17|Num=151| Name=Pope| Seat=Golconda| Data2=1816| Data3=Gallatin and Johnson County |Data4=[[Nathaniel Pope]] (1784–1850), early Delegate from [[Illinois Territory]] to Congress and judge on the [[United States District Court for the District of Illinois]] | Population=3703| Area=368| Size=60px}}
{{Countyrow|N=17|Num=153| Name=Pulaski| Seat=Mound City| Data2=1843| Data3=Alexander and Johnson County |Data4=Gen. [[Casimir Pulaski]] (1745–1779), [[Polish American]] general of cavalry in the [[American Revolutionary War]] | Population=4879| Area=199| Size=60px}}
{{Countyrow|N=17|Num=155| Name=Putnam| Seat=Hennepin| Data2=1825| Data3=Fulton County |Data4=Gen. [[Israel Putnam]] (1718–1790), commander of American forces at the [[Battle of Bunker Hill]] | Population=5633| Area=160| Size=60px}}
{{Countyrow|N=17|Num=157| Name=Randolph| Seat=Chester| Data2=1795| Data3=St. Clair County |Data4=[[Edmund Randolph]] (1753–1813), first [[Attorney General of the United States]], and briefly [[United States Secretary of State]] | Population=29903| Area=575| Size=60px}}
{{Countyrow|N=17|Num=159| Name=Richland| Seat=Olney| Data2=1841| Data3=Clay and Lawrence County |Data4=[[Richland County, Ohio]], itself named for its rich [[soil]] | Population=15362| Area=360| Size=60px}}
{{Countyrow|N=17|Num=161| Name=Rock Island| Seat=Rock Island| Data2=1831| Data3=Jo Daviess County |Data4=[[Rock Island Arsenal|Rock Island]] | Population=142731| Area=427| Size=60px}}
{{Countyrow|N=17|Num=165| Name=Saline| Seat=Harrisburg| Data2=1847| Data3=Gallatin County |Data4=The [[Saline River (Illinois)|Saline River]] and salt springs in the county | Population=22826| Area=379| Size=60px}}
{{Countyrow|N=17|Num=167| Name=Sangamon| Seat=Springfield| Data2=1821| Data3=Madison and Bond County |Data4=[[Sangamon River]] | Population=194345| Area=868| Size=60px}}
{{Countyrow|N=17|Num=169| Name=Schuyler| Seat=Rushville| Data2=1825| Data3=Pike and Fulton County |Data4=Gen. [[Philip Schuyler]] (1733–1804), [[American Revolutionary War]] general and [[United States Senator]] from [[New York (state)|New York]] | Population=6750| Area=437| Size=60px}}
{{Countyrow|N=17|Num=171| Name=Scott| Seat=Winchester| Data2=1839| Data3=Morgan County |Data4=[[Scott County, Kentucky]], itself named after [[Charles Scott (governor)|Charles Scott]] | Population=4880| Area=250| Size=60px}}
{{Countyrow|N=17|Num=173| Name=Shelby| Seat=Shelbyville| Data2=1827| Data3=Fayette County |Data4=[[Isaac Shelby]] (1750–1826), soldier in the [[American Revolutionary War]] and [[War of 1812]]; first and fifth [[Governor of Kentucky]] | Population=20542| Area=758| Size=60px}}
{{Countyrow|N=17|Num=163| Name=St. Clair| Seat=Belleville| Data2=1790| Data3=original two counties |Data4=[[Arthur St. Clair]] (1737–1818), major general in the [[American Revolutionary War]] and first Governor of the [[Northwest Territory]] | Population=251149| Area=657| Size=60px}}
{{Countyrow|N=17|Num=175| Name=Stark| Seat=Toulon| Data2=1839| Data3=Knox and Putnam County |Data4=Gen. [[John Stark]] (1728–1822), general in the [[American Revolutionary War]], called the "Hero of [[Battle of Bennington|Bennington]]" | Population=5272| Area=288| Size=60px}}
{{Countyrow|N=17|Num=177| Name=Stephenson| Seat=Freeport| Data2=1837| Data3=Jo Daviess and Winnebago County |Data4=[[Benjamin Stephenson (politician)|Benjamin Stephenson]] (1769–1822), representative of [[Illinois Territory]] in the [[United States Congress]] from 1814 to 1816 | Population=43088| Area=564| Size=60px}}
{{Countyrow|N=17|Num=179| Name=Tazewell| Seat=Pekin| Data2=1827| Data3=Sangamon County |Data4=[[Littleton Waller Tazewell]] (1774–1860), United States Senator from (and later governor of) [[Virginia]] | Population=129821| Area=646| Size=60px}}
{{Countyrow|N=17|Num=181| Name=Union| Seat=Jonesboro| Data2=1818| Data3=Johnson County |Data4=The union of the [[United States]] | Population=16886| Area=413| Size=60px}}
{{Countyrow|N=17|Num=183| Name=Vermilion| Seat=Danville| Data2=1826| Data3=Edgar County |Data4=The [[Vermilion River (Wabash River tributary)|Vermilion River]] | Population=71106| Area=898| Size=60px}}
{{Countyrow|N=17|Num=185| Name=Wabash| Seat=Mount Carmel| Data2=1824| Data3=Edwards County |Data4=The [[Wabash River]] | Population=10979| Area=223| Size=60px}}
{{Countyrow|N=17|Num=187| Name=Warren| Seat=Monmouth| Data2=1825| Data3=Schuyler County |Data4=[[Joseph Warren]] (1741–1775), played a role in [[American Patriot]] movements, a prominent early fatality in the [[American Revolutionary War]] | Population=16217| Area=542| Size=60px}}
{{Countyrow|N=17|Num=189| Name=Washington| Seat=Nashville| Data2=1818| Data3=St. Clair County |Data4=[[George Washington]] (1732–1799), commander-in-chief of American forces in the [[American Revolutionary War]] and first [[President of the United States]] | Population=13535| Area=562| Size=60px}}
{{Countyrow|N=17|Num=191| Name=Wayne| Seat=Fairfield| Data2=1819| Data3=Edwards County |Data4=Gen. "Mad" [[Anthony Wayne]] (1745–1796), [[major general]] in the United States Army in the [[American Revolutionary War]] and the [[Northwest Indian War]] | Population=15928| Area=713| Size=60px}}
{{Countyrow|N=17|Num=193| Name=White| Seat=Carmi| Data2=1815| Data3=Gallatin County |Data4=[[Isaac White (militia colonel)|Isaac White]] (1776–1811), resident of Illinois who enlisted in the Indiana militia and was killed at the [[Battle of Tippecanoe]] | Population=13395| Area=494| Size=60px}}
{{Countyrow|N=17|Num=195| Name=Whiteside| Seat=Morrison| Data2=1836| Data3=Jo Daviess and Henry County |Data4=[[Samuel Whiteside]] (1783–1868), state legislator and militia leader | Population=54657| Area=684| Size=60px}}
{{Countyrow|N=17|Num=197| Name=Will| Seat=Joliet| Data2=1836| Data3=Cook and Iroquois County |Data4=[[Conrad Will (politician)|Conrad Will]] (1779–1835),<ref>{{cite news|last=Matile|first=Roger|title=Reflections: Was Dr. Conrad Will really worth his salt?|url=http://www.ledgersentinel.com/article.asp?a=5216|access-date=October 11, 2011|newspaper=Ledger-Sentinel|date=June 22, 2006}}</ref> physician, local businessman and longtime member of the state legislature | Population=708583| Area=835| Size=60px}}
{{Countyrow|N=17|Num=199| Name=Williamson| Seat=Marion| Data2=1839| Data3=Franklin County |Data4=[[Williamson County, Tennessee]], which was named for [[Hugh Williamson]] | Population=67064| Area=420| Size=60px}}
{{Countyrow|N=17|Num=201| Name=Winnebago| Seat=Rockford| Data2=1836| Data3=Jo Daviess County |Data4=[[Winnebago (tribe)|Winnebago]] Native Americans | Population=283790| Area=513| Size=60px}}
{{Countyrow|N=17|Num=203| Name=Woodford| Seat=Eureka| Data2=1841| Data3=Tazewell and McLean County |Data4=[[Woodford County, Kentucky]], which was named for [[William Woodford]] | Population=38290| Area=527| Size=60px}}
|}

==Defunct counties==
* Dane County was renamed in 1840 to the current [[Christian County, Illinois|Christian County]].
* The original Knox County, Illinois, became extinct with the formation of the [[Illinois Territory]] in 1809 - or, more precisely, it became [[Knox County, Indiana]]. The modern [[Knox County, Illinois]] was formed much later and was not a part of the original Knox County.

==See also==
{{Portal|Illinois}}
* [[List of census-designated places in Illinois]]
* [[List of cities in Illinois]]
* [[List of Illinois townships]]
* [[List of precincts in Illinois]]
* [[List of towns and villages in Illinois]]
* [[List of unincorporated communities in Illinois]]
* [[National Association of Counties]]
* [[National Register of Historic Places listings in Illinois]]

==Notes==
{{Reflist}}

==References==
* {{cite web|last=Hébert|first=Michael L.|title=Illinois County Boundaries: 1790 - Present|url=http://maps.ilgw.org/|publisher=ILGenWeb|year=2012|access-date=June 16, 2010|archive-url=https://web.archive.org/web/20160126184839/http://maps.ilgw.org/|archive-date=January 26, 2016|url-status=dead}}
* {{cite web|last=White|first=Jesse|title=Origin and Evolution of Illinois Counties |url=http://www.cyberdriveillinois.com/publications/pdf_publications/ipub15.pdf|publisher=[[Illinois Secretary of State]]|author-link=Jesse White (politician)|date=March 2010}}
* {{cite web |last1=writer |first1=Statf |author-link1= |collaboration= |df= |year=2022 |location= |title=ILLINOIS COUNTY GOVERNMENT OVERVIEW |url=https://ce.naco.org//app/profiles/CountyGov/CountyGov_17000.pdf |url-access= |format= |department= |website=ce.naco.org |type= |language= |edition= |agency=National Association of Counties (NACo) |arxiv= |asin= |asin-tld= |bibcode= |bibcode-access= |biorxiv= |citeseerx= |doi= |doi-access= |doi-broken-date= |eissn= |hdl= |hdl-access= |isbn= |ismn= |issn= |jfm= |jstor= |jstor-access= |lccn= |medrxiv= |mr= |oclc= |ol= |ol-access= |osti= |osti-access= |pmc= |pmc-embargo-date= |pmid= |rfc= |sbn= |ssrn= |s2cid= |s2cid-access= |zbl= |id= |url-status= |archive-url= |archive-format= |archive-date= |access-date=January 3, 2025 |quote-page= |quote-pages= |quote= |ref= }}

==External links==
{{Commons category|Counties of Illinois}}
* [https://web.archive.org/web/20020802223743/http://www.census.gov/geo/www/gazetteer/places2k.html Census 2000 U.S. Gazetteer Files]
* [http://www.ilcounty.org/ Illinois Association of County Board Members]
* [http://www.iacoonline.org/ Illinois Association of County Officials]
* [http://www.ilcma.org/ Illinois City/County Management Association]

{{Illinois}}
{{U.S. Counties}}

[[Category:Illinois counties|*]]
[[Category:Illinois geography-related lists|Counties]]
[[Category:Lists of counties of the United States by state|Illinois, counties in]]