William Pratt was born on 23 November 1887 at 36 Forest Hill Road, Camberwell, London, England. Pratt himself stated that he was born in Dulwich, which is nearby in London. His parents were Edward John Pratt, Jr. and Eliza Sarah Millard. His maternal grandparents were James Millard and Eliza Julia Edwards, a sister of Anna Leonowens (whose tales about life in the royal court of Siam (now Thailand) were the basis of the musical The King and I). The two sisters may have had some Anglo-Indian ancestry.
Pratt spent his childhood years in Enfield, in the County of Middlesex. He was the youngest of nine children, and following his mother’s death was brought up by his elder siblings. He received his early education at Enfield Grammar School, and later at the private schools of Uppingham School and Merchant Taylors’ School. After this he attended King’s College London where he took studies aimed at a career with the British Government’s Consular Service. However, in 1909 he left university without graduating and drifted, departing England for Canada, where he worked as a farm labourer and did various odd itinerant jobs until happening into acting. His brother, Sir John Thomas Pratt, became a distinguished British diplomat.
He was bow-legged, had a lisp, and stuttered as a young boy. He conquered his stutter, but not his lisp, which was noticeable all through his following career.