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
Acting prime minister to an outgoing Prime Minister following an electoral defeat, and who by convention does not implement new policies or an interim Prime Minister who is appointed to perform a similar role to a caretaker Prime Minister, but who is typically not a Prime Minister at the time of being appointed. Within a Commonwealth nation, the office of Acting Prime Minister, like that of Prime Minister is found only in convention, and is not legislated. The Department of Prime Minister and Cabinet, or an equivalent ministry, drafts, reviews and publishes any formal conventions (though these are expressly non binding in a