Known as 'Hammie', Gault was born in England, the only son of a native of Strabane, Andrew Frederick Gault (1833–1903), of Rokeby in Montreal's Golden Square Mile; and his wife Louise Sarah Harman (1847–1937), daughter of Henry B. Harman, of Surrey. His middle name, which he used as his first, was for his paternal grandmother's family, the Hamiltons of Fintra House, near Killybegs, Co. Donegal. His uncles included the Hon. Matthew Hamilton Gault and Sir James Welsh Skelton, and when his father (the "Cotton King of Canada") died in 1915, he, his mother and sister inherited just over $1.3 million each.