Sir William McMahon (23 February 1908 – 31 March 1988) was the 20th Prime Minister of Australia. He was elected to the Australian parliament in 1949, and stayed as Member for Lowe, in Sydney, for 33 years. He challenged John Gorton for the leadership of the Liberal Party of Australia in 1971 and won. He was 63 years old. He lost the next election in December 1972 to Gough Whitlam of the Australian Labor Party. This made him the first Prime Minister not to win an election since Labor's Ben Chifley (who lost in 1949). McMahon was married to (Lady) Sonia McMahon from 1965 until his death from cancer. Actor Julian McMahon is their son.