The Prime Minister's Office (PMO) is a central agency that came into its own in the late 1960s. It differs from its counterparts in that it is staffed with temporary political appointees rather than full-time, career civil servants and has no statutory base, its budget being a component of the estimates for the Privy Council Office. The Prime Minister determines the PMO's organization and role; its functions derive from the prime minister's political responsibilities as party leader rather than as head of government, though in practice the division between these responsibilities is not clear, thereby providing opportunities for the PMO to trespass on the more purely administrative preserves of other central agencies.

The PMO is responsible for press and public relations, the PM's large correspondence, speaking engagements, etc; it advises on candidates for appointment to the numerous order-in-council appointees, eg, directorships on crown corporations, members of regulatory commissions, on which the PM's recommendation is essential and decisive; it maintains contact with the party's officials outside the legislature and with the party caucus in the legislature; it generally serves as a listening post and a "gate-keeper" determining which matters will be brought to the PM's attention and ensuring that the political dimensions of public policies are not overlooked by the permanent bureaucracy. There is potential for overlap and competition with the Privy Council Office. The expansion of the personnel and functions of the PMO, which coincided with the prime ministership of Pierre Trudeau, reflected the increasingly dominant role of the PM, as head of government and head of party, and has been perpetuated by Trudeau's successors.

The inherent tension between the political party-oriented role and the policy advisory functions of his PMO staff gave rise to contradictory conclusions that either the PMO is weak and fails to provide direction or else is too strong and trespasses on the turf of other central agencies - most notably the PCO.