thumb|243x243px|[[Wilfrid Laurier|Sir Wilfrid Laurier, the longest consecutively serving prime minister]]
The prime minister of Canada is the head of government of Canada. Since Canadian Confederation in 1867, there have been 24 prime ministers who have formed 30 Canadian ministries. The first prime minister, Sir John A. Macdonald, took office on July 1, 1867. The position does not have a set term of office and does not have term limits. Instead, prime ministers can stay in office as long as their government has the confidence of a majority in the House of Commons of Canada under the system of responsible government. Under this system, William Lyon Mackenzie King was Canada's longest-serving prime minister, holding office for a total of 21 years and 154 days over three non-consecutive terms.
The prime minister's term begins upon appointment by the governor general of Canada, usually after winning a general election. One prime minister can also succeed another with no election—usually, but not necessarily, when they are successive leaders of the same party. A prime minister stays in office until they resign, die or are dismissed by the governor general. Two prime ministers have died in office (Macdonald and Sir John Thompson). All others have resigned, either after losing an election, a parliamentary no confidence vote, or upon retirement. Theoretically, the monarch or governor general can dismiss a prime minister, but that has never happened.
The prime ministerial term is not tied directly to the term of the House of Commons, which the Constitution sets as a maximum of five years from the most recent general election. A prime minister takes office after winning an election and resigns after losing an election, but the term in office does not match up directly to the term of the Parliament. An incoming prime minister will normally take office a few weeks after the election, and an outgoing prime minister will usually stay in office for a few weeks after losing the election. The transition period and the date for the transfer of office are negotiated by the incoming and the outgoing prime ministers.
A prime minister who holds office in consecutive parliaments is not re-appointed as prime minister for each parliament, but rather serves one continuous term.
A majority government normally lasts around four years, since general elections for Parliament are normally held every four years. Minority governments generally last for a shorter period. The shortest minority government, Arthur Meighen's second government, lasted just under three months. John Turner and Kim Campbell both served short terms for similar reasons.
Of the other prime ministers who served short terms, Arthur Meighen, Joe Clark, and Paul Martin had their time in office cut short by the collapse of their minority governments and the subsequent election of the opposition party. Two prime ministers have served without ever sitting in the House of Commons during their time in office: Tupper and Turner.