William Lyon Mackenzie King was a dominant Canadian political figure who served as the tenth Prime Minister of Canada. He held the office for three non-consecutive terms from 1921 to 1930 and 1935 to 1948, making him the longest-serving prime minister in British Commonwealth history. A member of the Liberal Party, he was instrumental in developing Canada's social safety net and asserting its autonomy from the United Kingdom. King's leadership guided the nation through the Great Depression and the Second World War, defining modern Canada in the process.