William Lyon Mackenzie King PC OM CMG (December 17, 1874 - July 22, 1950) was a Canadian lawyer, economist, university professor, civil servant, journalist, and politician. He served as the tenth Prime Minister of Canada from December 29, 1921, to June 28, 1926; September 25, 1926, to August 6, 1930; and October 23, 1935, to November 15, 1948. With over 21 years in the office, he was the longest serving Prime Minister in British Commonwealth history. He is commonly known either by his full name or as Mackenzie King. Mackenzie was one of his given names, not part of his surname, but he was never publicly referred to as simply "William King." Friends and family called him by his nickname, "Rex
