He studied history under Mommsen and Droysen, and much of the time he worked as private secretary to George Bancroft, United States Minister at Berlin.

He received a doctorate from the University of Leipzig, with a dissertation entitled The Poet Labid: His Life, Times, and Fragmentary Writings, which was published in 1877.

The published version of Sloane's dissertation specifically mentions that he studied under Fleischer, Krehl, and Loth [de] at Leipzig.

Sloane was a professor of Latin (1877–1883) and subsequently History (1883–1896) at Princeton University, when it was still known as the College of New Jersey.

William Milligan Sloane (the grandson) was a marine aviator and corporate lawyer who married Martha Chamberlin.