Still was born free in 1821, near Medford, in the Pine Barrens of New Jersey, the youngest of eighteen children. His father, Levin, had purchased his freedom and moved north from Maryland in 1807. His mother, charity, later escaped to join him there, leaving behind their two oldest, enslaved sons. Largely self-taught, William moved to Philadelphia in 1844, where he worked at various menial jobs until, in 1847, he was hired as a clerk and a janitor by the Pennsylvania Society for the Abolition of Slavery, at a salary of three dollars and seventy-five cents per week.