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
Historical rankings of Prime Ministers of Australia survey (and was judged to be the overall winner), although Alfred Deakin was nominated as the greatest prime minister by the most respondents. The four shortest-serving prime ministers were excluded from consideration, as well as the serving prime minister, Paul Keating. The total number of points was not published for prime ministers ranked outside the ten (except for those who received no points), only the order. In 2001, to commemorate the centenary of federation, "The Australian Financial Review" asked six historians to answer the question "who were the best five and who were the worst five prime ministers since 1901?".