Not outlined in any constitutional document, the prime minister is appointed by the monarch's representative, the governor general, and the office exists per long-established convention.

Canadian prime ministers are appointed to the Privy Council and styled as the Right Honourable (French: le très honorable),[note 4] a privilege maintained for life.

[8] Justin Trudeau, the current prime minister, took office on November 4, 2015, following the 2015 federal election, wherein his Liberal Party won a majority of seats.

[18] By the conventions of responsible government, the foundation of parliamentary democracy, the governor general will call to form a government the individual most likely to receive the support, or confidence, of a majority of the directly elected members of the House of Commons;[19] as a practical matter, this is often the leader of the party, or a coalition of parties,[20][21] whose members form a majority, or a very large plurality, of seats in the House of Commons.

[22] No document is needed to begin the appointment; a party leader becomes prime minister-designate as soon as he accepts the governor general's invitation to form a government.

[23] If the leader of the opposition is unable or unwilling to form a government,[note 5] the governor general can consult whomever she wishes.

[26] Both, in their roles as government leader in the Senate, succeeded prime ministers who had died in office—John A. Macdonald in 1891 and John Sparrow David Thompson in 1894.

[32] As such, the prime minister, supported by the Office of the Prime Minister (PMO), controls the appointments of many key figures in Canada's system of governance, including the governor general, the Cabinet, justices of the Supreme Court, senators, heads of Crown corporations, ambassadors and high commissioners, the provincial lieutenant governors, and approximately 3,100 other positions.

"[37] Indeed, the position has been described as undergoing a "presidentialization",[33][38] to the point that its incumbents publicly outshine the actual head of state (and prime minister's spouses are sometimes referred to as First Lady of Canada[39][40]).

[43] It has been theorized that such is the case in Canada as its Parliament is less influential on the executive than in other countries with Westminster parliamentary systems; particularly, Canada has fewer MPs, a higher turnover rate of MPs after each election, and a US-style system for selecting political party leaders, leaving them accountable to the party membership rather than caucus (as is the case in the UK).

Caucuses may choose to follow these rules, though the decision would be made by recorded vote, thereby subjecting the party's choice to public scrutiny.

[note 8] Near the end of her time as governor general, Adrienne Clarkson stated: "My constitutional role has lain in what are called 'reserve powers': making sure that there is a prime minister and a government in place, and exercising the right 'to encourage, to advise, and to warn'[...] Without really revealing any secrets, I can tell you that I have done all three.

Accordingly, several carried the prefix Sir before their name; of the first eight prime ministers of Canada, only Alexander Mackenzie refused the honour of a knighthood from Queen Victoria.

The Crown in right of Canada (but not the Crown in right of the United Kingdom, which has periodically bestowed such Imperial honours on such citizens) has since adopted this policy generally, such that the last prime minister to be knighted near appointment was Robert Borden, who was the prime minister at the time the Nickle Resolution was debated in the House of Commons (and was knighted before the resolution).

[60][61][62][63][64] Joe Clark,[60] Pierre Trudeau,[61] John Turner,[62] Brian Mulroney,[63] Kim Campbell,[64] Jean Chrétien[65] and Paul Martin[66] were granted arms with the augmentation.

In the decades following Confederation, it was common practice to refer to the prime minister as Premier of Canada,[69][70][71] a custom that continued during the First World War, around the time of Robert Borden's premiership.

Meighen was also appointed to the Senate following his second period as prime minister, but resigned his seat to seek re-election and moved to private enterprise after failing to win a riding.