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
Records of Prime Ministers of the United Kingdom March 2036 (his 93rd birthday), he will surpass Callaghan's record and become the longest-lived Prime Minister. The shortest-lived Prime Minister was the Duke of Devonshire, who was born on 8 May 1720 and died on 2 October 1764 at the age of 44 years and 147 days. The Prime Minister who lived the longest after leaving office for the final time was the Duke of Grafton, who left office on 28 January 1770 and died on 14 March 1811, a total of 41 years and 45 days. In recent years, the Prime Minister who lived the longest after leaving office