See also the chapter on prime minister biographies.
In the first 90 years following Confederation, Canada technically had no formal office of prime minister. The highest institution in the executive branch was simply the cabinet, and whichever cabinet minister was leader of the ruling party was called head of cabinet or head of government, and informally referred to as Canada’s prime minister or premier, though officially he was often just called by whatever cabinet job he had. For example, in the 1871 Treaty of Washington, which Prime Minister Macdonald signed, he is simply listed as “Minister of Justice and Attorney General of Her Majesty’s Dominion of Canada.”
It was not until 1957 that someone served as prime minister without also holding another cabinet job. Likewise, prior to 1957 there was no inauguration for prime minister; record keepers simply consider a pre-1957 prime minister’s term to have begun when he was sworn into the cabinet job he got when his party assumed power.
Louis St. Laurent served briefly as prime minister and no other office before losing the 1957 election to John Diefenbaker, who took office as foreign minister when he became prime minister on June 21, 1957. However, a few months later, on September 12, Diefenbaker, like St. Laurent, also stepped down from any other cabinet position and governed as prime minister and no other position for most of the remainder of his term.