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
Prime Minister of Denmark "Council President" (). Carl Christian Hall became the first Prime Minister/Council President to lead a political party (the National Liberal Party). The modern Prime Minister's Office was founded on 1 January 1914, when the Council Presidium was established as a department under the Prime Minister, when it had previously existed as an informal council gathered by the Prime Minister. The title of the Prime Minister changed again in 1918 under the Premiership of Carl Theodor Zahle, becoming titled the "Minister of State" (in-line with its Scandinavian neighbours, Norway and Sweden), which it remains to this day. By the mid-nineteenth century