Educated at Sydney Grammar School and the University of Sydney, where he studied law, he practised as a  solicitor before enlisting in the 2nd AIF in early 1940. He served in Australia before being discharged in 1945 with the  rank of Major.

McMahon entered parliament in 1949, winning the Sydney House of Representatives seat of Lowe for the Liberal  Party. This was the election that returned the Liberals to government – in coalition with the Country Party – for a  period in power that lasted 23 years. McMahon served on the backbench for only 18 months before promotion to the ministry as Minister for the service departments of Navy and Air (1951–54). Throughout the 1950s and 1960s he served successively as Minister for Social Services (1954–56), Primary Industry (1956–58), Labour and National Service (1958–66), Treasurer (1966–69), External Affairs (1960–70) and Foreign Affairs (1970–71).

McMahon served as Deputy Leader of the Liberal Party under Prime Ministers Harold Holt and John Gorton between 1966 and 1971. In March 1971, one month after McEwen’s retirement from parliament, McMahon succeeded in a leadership challenge against Gorton and became Prime Minister.

The McMahon government was responsible for a number of initiatives – it was the first to create a department solely responsible for Aboriginal Affairs, and it effectively ended Australia’s military commitment to Vietnam. After 23 years of coalition government, McMahon lost the 1972 general election.  McMahon resigned as party leader after the election. He remained in parliament until 1982 but did not have a front bench role in later Liberal governments. McMahon was appointed a member of the Privy Council in 1966, made a Companion of Honour in 1972, and knighted in 1977.

Initiated: Lodge University of Sydney No. 544 UGL of NSW on 22 March 1974.