Schlesinger was born in New York City and attended public schools there.<ref name=anb>Template:Cite ANB</ref> He graduated from the College of the City of New York in 1890. He then worked as a surveyor, becoming a special student in astronomy at Columbia in 1894. In 1896, he received a fellowship which enabled him to study full-time,<ref name=anb/> and he received a PhD in 1898. After his graduation, he spent the summer at Yerkes Observatory as a volunteer assisting director George Ellery Hale.<ref name=dab>Template:Cite DAB</ref>
Asked how to say his name, he told The Literary Digest "The name is so difficult for those who do not speak German that I am usually called sles'in-jer, to rhyme with messenger. It is, of course, of German origin and means 'a native of Schlesien' or Silesia. In that language the pronunciation is shlayzinger, to rhyme with singer."<ref>Charles Earle Funk, What's the Name, Please?, Funk & Wagnalls, 1936.</ref>
He married Eva Hirsch in 1900 while in Ukiah. They had one child, Frank Wagner Schlesinger, who later directed planetariums in Philadelphia and Chicago. His wife died in 1928, and in 1929 he married Mrs. Katherine Bell (Rawling) Wilcox.<ref name=dab/>
