William Lyon Mackenzie King OM CMG PC was a Canadian statesman and politician who served as the 10th 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 during both the interwar period and World War II, from the 1920s through the 1940s. He is best known for his leadership of Canada throughout the Second World War when he mobilized Canadian money, supplies and volunteers to support Britain while boosting the economy and maintaining morale on the home front. With a total of 21 years and 154 days in office, he remains the longest-serving prime minister in Canadian history. Trained in law and social work, he was keenly interested in the human condition, and played a major role in laying the foundations of the Canadian welfare state.
King acceded to the leadership of the Liberal Party in 1919, following the death of Sir Wilfrid Laurier. Taking the helm of a party bitterly torn apart during the First World War due to the Conscription Crisis of 1917, he unified factions of the Liberal Party, leading it to victory in the 1921 election.