William Lyon Mackenzie King, Canadian economist and politician, 10th Prime Minister of Canada (born 1874)

William Lyon Mackenzie King served as the tenth prime minister of Canada for three non-consecutive terms from 1921 to 1926, 1926 to 1930, and 1935 to 1948. A Liberal, he was the dominant politician in Canada from the early 1920s to the late 1940s. With a total of 21 years and 154 days in office, he remains the longest-serving prime minister in Canadian history.

The prime minister of Canada is the head of government of Canada. Under the Westminster system, the prime minister governs with the confidence of a majority of the elected House of Commons; as such, the prime minister typically sits as a member of Parliament (MP) and leads the largest party or a coalition of parties. As first minister, the prime minister selects ministers to form the Cabinet.