William McMahon, Australian lawyer and politician, 20th Prime Minister of Australia (d. 1988)
Sir William McMahon (23 February 1908 – 31 March 1988) 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. He was appointed Treasurer in the Holt Government, and over the following three years oversaw a large reduction in the national deficit. After Holt's death in 1967, McMahon wished to contest the Liberal leadership but had his candidacy vetoed by John McEwen, the leader of the Country Party. The new prime minister was John Gorton. McMahon initially continued on as Treasurer in the Gorton Government, but in 1969 was demoted to Minister for External Affairs after an unsuccessful challenge for the leadership. He eventually forced Gorton's resignation in early 1971, winning a vote against Billy Snedden.

McMahon became prime minister at the age of 63, and remains the oldest non-interim prime minister to take office. His government has been described by the Australian Dictionary of Biography as "a blend of cautious innovation and fundamental orthodoxy". It continued many of the policies of its immediate predecessors, such as the phased withdrawal of Australian troops from Vietnam. In its final year it faced high inflation and unemployment. Gough Whitlam's Labor Party defeated McMahon at the 1972 federal election, ending 23 consecutive years of Coalition rule. No other Australian prime minister has served for longer without winning a general election. He resigned the Liberal leadership, but remained in parliament until 1982 as a backbencher.

McMahon has been described as one of Australia's worst prime ministers by Australian political scientists and historians, and after leaving office several of his former colleagues openly criticised his leadership style and personal character. However, Whitlam acknowledged him as "an extraordinarily skilful, resourceful and tenacious politician", and credited him with having prevented a larger margin of defeat in 1972.