William Surrey Hart was born on December 6, 1864 in Newburgh, New York, United States. He was the second son and second of eight children of British parents, Nicolas Hart and Rose (McCauley) Hart. His father had emigrated from Liverpool; his mother, born in northern Ireland, had grown up in Newburgh.
Nicolas Hart, a miller who specialized in locating sites for grain mills and supervising their construction, headed west with his family soon after William's birth; never staying long in one place, he moved from town to town across the plains states, from Illinois to the Dakotas. Illness twice took William's mother back to Newburgh for protracted periods, but William spent much time with his father in the West. He lived alongside the Sioux in Minnesota and the Dakotas, learning their language, and in the frontier towns of Kansas during the days of the cattle drives. In his later life and work he tried to re-create the authentic flavor of experience in what he called "the unbroken West. "
Hart's father returned to the East in the middle 1870s, and the reunited family moved to West Farms in the suburbs of New York and then into the city.