Answer:
William Lyon Mackenzie King is the longest serving Prime Minister in Canadian history, and indeed, in the history of the British Commonwealth. He served a crucial role in developing Canada's autonomy from Britain, and by extension, the autonomy of the other dominions as well. He was also the Prime Minister of Canada throughout WWII. King is particularly known for his involvement in the King-Byng affair, and for communing with the spirits of the deceased (including those of his mother and his dogs). His face is featured on the Canadian $50 bill.