 He was the third of five children born to solicitor William Daniel McMahon and Mary (née Walder), daughter of a sailmaker; an older brother predeceased him.His father, a Catholic, had a reputation as a heavy drinker and habitual gambler; his mother, an Anglican, was of English and Irish descent.McMahon's paternal grandfather, James "Butty" McMahon, was born in County Clare, Munster, Ireland, and married Mary Coyle of County Fermanagh, Ulster (in present-day Northern Ireland).