The longest-serving Prime Minister of Canada was William Lyon Mackenzie King, who held office for a total of over 22 years across three non-consecutive terms in the 20th century. King is notable for his leadership during critical periods, including the Great Depression and World War II. His tenure was marked by significant social and political changes, including the development of Canada's welfare state and its increasing autonomy from Britain. His long service left a profound impact on the Canadian political landscape.