He was educated at Abootshome on the North Shore, then at the exclusive Sydney Grammar School, before studying Law and Economics at Sydney University.
McMahon served on the homefront in World War II, after failing a 1940 medical examination because he suffered from deafness.
In 1949, a friend of his was seeking pre-selection, who could not attend a function. McMahon gave a speech on his behalf and it was so impressive he was urged to run for Parliament himself.
McMahon was elected to the seat of Lowe, when Robert Menzies swept to power. In 1951, he was appointed Minister for the Navy and Air. In 1954, he became Minister for Social Services.
In 1956, he became Minister for Primary Industries, and clashed with Arthur Fadden and John McEwen. By 1958, McMahon moved to the portfolio of National Service.
McMahon married Sydney socialite Sonya Hopkins in 1965.
Harold Holt awared McMahon the position of Treasurer in 1966, and he also became Deputy Leader that year.
When Harold Holt disappeared in December 1967, it was expected McMahon would become Prime Minister, however Country Party leader, John McEwen refused to work with him, thus paving the way for Senator John Gorton to become Prime Minister.
In March 1971, Gorton used his casting vote to step down, when a Leadership ballot between himself and McMahon drew even. Billy McMahon was very well liked by Sir Frank Packer, who live around the corner from him in Bellevue Hill. McMahon was Packer's man.
McMahon faced the electorate on 2 December 1972, after 23 years of Coalition rule, against the charismatic Gough Whitlam. Although McMahon's campaign and style were criticised, many overlooked that Whitlam won with a tiny majority, small enough to see this miniaturised in 1974 and paving the way for a landslide victorious return in 1975.
Sir William McMahon continued to serve in his electorate of Lowe until his resignation in Janaury 1982.
He fought a long battle with cancer and died on 31 March 1988.
Responsibility for Election Commentary taken by A.Molloy, PO Box 669, Double Bay, 2028.