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
Australian home front during World War II the plan. Menzies announced to his Cabinet that he thought he should resign and advise the Governor-General to invite Curtin to form Government. The Cabinet instead insisted he approach Curtin again to form a war cabinet. Unable to secure Curtin's support, and with an unworkable parliamentary majority, Menzies resigned as prime minister and leader of the UAP on 29 August 1941. He was succeeded as prime minister by Fadden, the leader of the Country Party, who held office for a month. Billy Hughes, then aged 79, replaced Menzies as leader of the UAP. The two independents crossed the floor, bringing