Sir William McMahon GCMG CH was an Australian politician who served as the 20th Prime Minister of Australia, in office from 1971 to 1972 as leader of the Liberal Party. He was a government minister for over 21 years, the longest continuous ministerial service in Australian history.
McMahon was born and raised in Sydney, and worked as a commercial lawyer before entering politics. He served in the Australian Army during World War II, reaching the rank of major. After the war's end he returned to university to complete an economics degree. McMahon was elected to the House of Representatives at the 1949 federal election. Robert Menzies promoted him to the ministry in 1951 and added him to cabinet in 1956. He held several different portfolios in the Menzies Government, most notably as Minister for Labour and National Service from 1958 to 1966. In that capacity he oversaw the reintroduction of conscription in 1964.
In 1966, Menzies retired and was replaced as prime minister by Harold Holt. McMahon then succeeded Holt as deputy leader of the Liberal Party.