High Quality Content by WIKIPEDIA articles! William Lyon Mackenzie King, PC, OM, CMG (December 17, 1874 ? July 22, 1950) was a Canadian lawyer, economist, university professor, consultant, civil servant, journalist, teacher, 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 an accumulated total of over 21 years in office, he was the longest-serving Prime Minister in British Commonwealth history.