William Lyon Mackenzie King was a prominent Canadian politician who served as the Prime Minister of Canada for over 21 years, making him the longest-serving prime minister in Canadian history. He was born on December 17, 1874, and played a significant role in shaping Canada during the early to mid-20th century, particularly during World War I and World War II. King was known for his leadership style, which emphasized compromise and coalition-building. He was a member of the Liberal Party of Canada and focused on social welfare policies, economic recovery, and national unity. His tenure included the establishment of key institutions and programs that contributed to Canada's development as a modern nation.