He is commonly known either by his full name or as Mackenzie King. (Mackenzie was one of his given names, not part of his surname.) In his public career he was never referred to as simply "William King".
King worked as a newspaper reporter for the Toronto Globe while studying at the University of Toronto. He was first elected to Parliament as a Liberal in a 1908 by-election, and was re-elected in a 1909 by-election following his appointment as Canada's first Minister of Labour. He lost his seat in the 1911 general election, which saw the Conservatives defeat his Liberals.
Following his defeat, he went to the United States to work for the Rockefeller family, assisting them in labour relations. He returned to Canada to run in the 1917 election, which focused almost entirely on the conscription issue, and lost again, due to his opposition to conscription, which was supported by the majority of English Canadians.
In 1919, he was elected leader at the first Liberal leadership convention, and soon returned to parliament in a by-election. King remained leader until 1948. In the 1921 election, his party defeated Arthur Meighen and the Conservatives, and he became Prime Minister.
In his first term as Prime Minister, he was opposed by the Progressive Party, which did not support trade tariffs. King called an election in 1925, in which the Conservatives won the most seats, but not a majority in the House of Commons. King held onto power with the support of the Progressives. Soon into his term, however, a bribery scandal in the Department of Customs was revealed, which led to more support for the Conservatives and Progressives, and the possibility that King would be forced to resign. King asked Governor General Lord Byng to dissolve Parliament and call another election, but Byng refused, the only time in Canadian history that the Governor General has exercised such a power. King resigned, and Byng asked Meighen to form a new government. When Meighen's government was defeated in the House of Commons a short time later, however, Byng called a new election in 1926.