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
Deputy Prime Minister of Australia leader of the Country Party (subsequently the National Party). That continues to be case when the Coalition is in government. In the case of Labor governments, the party's deputy leader was and continues to be the Deputy Prime Minister. On 19 December 1967, John McEwen, the long-serving leader of the Country Party in the Coalition government, was sworn in as interim Prime Minister following the sudden death in office of Prime Minister Harold Holt. (There was discussion that deputy Liberal leader and Treasurer William McMahon should assume the office. McMahon had planned a party room meeting on 20 December to