Mandate
The Canadian prime minister serves at Her Majesty's pleasure, meaning the post does not have a fixed term; once appointed and sworn in by the governor general, the prime minister remains in office until he or she resigns, is dismissed, or dies. The lifespan of parliament is limited by the constitution to five years and, though the governor general may still, on the advice of the prime minister, dissolve parliament and issue the writs of election prior to the date mandated by the Canada Elections Act; the King-Byng Affair was the only time since Confederation that the viceroy deemed it necessary to refuse his prime minister's request for a general vote.
Following parliamentary dissolution, the prime minister must run in the resulting general election if he or she wishes to maintain a seat in the House of Commons. Should the prime minister's party subsequently win, it is unnecessary to re-appoint the prime minister or again swear him or her into office. If, however, an opposition party wins a majority of seats in the House of Commons, the prime minister may resign or be dismissed by the governor general. Should the prime minister's party achieve a minority while an opposition party wins a plurality—i.e., more seats than any other party but less than a majority—the prime minister can attempt to maintain the confidence of the House by forming a coalition with other minority parties. This option has almost never been entertained in Canada, the last time being in 1925.
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