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.

King realized the necessity of World War II before Hitler invaded Poland in 1939, and actually began mobilizing on 25 Aug 1939, with full mobilization on 1 September. Unlike World War I, however, when Canada was automatically at war as soon as Britain joined, King asserted Canadian autonomy by waiting until September 10, when a vote in the House of Commons took place, to support the government's decision to declare war. During this time Canada was able to acquire weapons from the United States. Upon declaring war Canada would not be able to purchase weapons from the US, under the US policy then in force of not arming belligerents. This issue soon became a moot point as the American embargo was repealed in November 1939.

King's promise not to impose conscription contributed to the defeat of Maurice Duplessis's Union Nationale Quebec provincial government in 1939 and Liberals' re-election in the 1940 election. But after the fall of France in 1940, Canada introduced conscription for home service. Still, only volunteers were to be sent overseas. King wanted to avoid a repeat of the Conscription Crisis of 1917. By 1942, the military was pressing King hard to send conscripts to Europe. In 1942, King held a national plebiscite on the issue asking the nation to relieve him of the commitment he had made during the election campaign. He said that his policy was "conscription if necessary, but not necessarily conscription."

French Canadians voted overwhelmingly against conscription, but a majority of English Canadians supported it. French and English conscripts were sent to fight in the Aleutian Islands in 1943 - technically North American soil and therefore not "overseas" - but the mix of Canadian volunteers and draftees found the Japanese had fled before their arrival. Otherwise, King continued with a campaign to recruit volunteers, hoping to address the problem with the shortage of troops caused by heavy losses in the Dieppe Raid in 1942, in Italy in 1943, and after the Battle of Normandy in 1944. In November 1944, the Government decided it was necessary to send conscripts to Europe. This led to a brief political crisis (see Conscription Crisis of 1944), but the war ended a few months later. Over 15,000 conscripts went to Europe, though only a few hundred saw combat.


King was extremely unpopular among Canadian servicemen and women during the war, who were pro conscription. His appearances at Canadian Army installations in Britain (and, after 6 June 1944, in Europe) were invariably greeted with boos and catcalls. When he was defeated after the war in his Prince Albert riding, the servicemen's vote was considered instrumental, and a sign was placed outside the town, similar to those that had been erected in The Netherlands, reading, "This Town Liberated by the Canadian Army."

from F.R. Scott, "W.L.M.K."

Dennis Lee, "William Lyon Mackenzie King"