Prime Minister of Australia prime minister, otherwise William McMahon63. Robert Menzies was the oldest person to ever be prime minister, leaving office at 71 years old. The longest-serving Prime Minister was Sir Robert Menzies, who served in office twice: from 26 April 1939 to 28 August 1941, and again from 19 December 1949 to 26 January 1966. In total Robert Menzies spent 18 years, 5 months and 12 days in office. He served under the United Australia Party and the Liberal Party respectively. The shortest-serving Prime Minister was Frank Forde, who was appointed to the position on 6 July 1945 after the death of
Prime Minister of Australia use it when he was no longer PM. Only one Prime Minister who had left the Federal Parliament ever returned. Stanley Bruce was defeated in his own seat in 1929 while Prime Minister but was re-elected to parliament in 1931. Other Prime Ministers were elected to parliaments other than the Australian federal parliament: Sir George Reid was elected to the UK House of Commons (after his term as High Commissioner to the UK), and Frank Forde was re-elected to the Queensland Parliament (after his term as High Commissioner to Canada, and a failed attempt to re-enter the Federal Parliament). From