Question: who is canada's longest serving prime minister
Answer: Prime Minister Incumbency Years in power Number of Mandates Remarks King ! William Lyon Mackenzie King 7001210000000000000 ♠ 21 years , 7002154000000000000 ♠ 154 days 1921 -- 1926 , 1926 -- 1930 , 1935 -- 1948 3 majority & 3 minority William Lyon Mackenzie King served for three full terms in majority governments ; two full terms in minority governments supported by opposition parties ; and one short term under a minority government cut short by the King - Byng Affair after which he was replaced by Arthur Meighen .