Who is the longest serving Prime Minister?
Australia’s longest serving Prime Minister is Robert Menzies. He held the office twice, from 1939 to 1941 and from 1949 to 1966. Altogether he was Prime Minister for over 18 years.
Sir Robert Menzies

John Curtin Prime Ministerial Library (JCPML00036/11)
Description
Sir Robert Gordon Menzies was Prime Minister of Australia from 1939 to 1941, and again from1949 to 1966.He is the longest-serving Prime Minister of Australia.