John S. Leary was born August 17, 1845 in Fayetteville, North Carolina to Matthew Leary and Julia Memriel. Matthew's father was an Irishman named Jeremiah Leary who married Sally Revels who was mixed Croatan Indian and black. Sally's father was Aaron Revels, and both Aaron Revels and Jeremiah Leary were veterans of the American Revolutionary War. Leary had at least three siblings, Lewis Sheridan Leary, Henrietta, and Michael Leary, Jr. Lewis was killed taking part in John Brown's raid on Harpers Ferry in 1859. Through Revels, Leary was a cousin to Hiram Rhodes Revels, the first African-American to serve in the United States Senate.