When Roblin moved to federal politics in 1967, Weir defeated Sterling Lyon and two other candidates to become the party's new leader. He was sworn in as Premier on November 27, 1967.
Weir represented a "rural populist" wing within the Manitoba Tories, and spoke for the party's more conservative members who had been marginalized during Red Tory Roblin's time as leader. Weir's government kept spending increases to a minimum and introduced a balanced budget without tax increases in 1968.[5] Weir was skeptical toward the concept of Medicare, and his government did not sign on to the program until 1969, one year after its introduction.[6] He also opposed the introduction of official bilingualism and attained national notoriety for his conflict with Canadian Prime MinisterPierre Trudeau over that issue.[7]
Weir also proposed reforms for the Senate of Canada. He called for all provinces to have an equal number of senators and for some Senate representatives to be appointed on the recommendation of provincial governments. He also recommended for the Senate to be given more powers, including the authority to ratify international treaties.[8]
Weir called four by-elections in early 1969, largely to test his government's popularity on the bilingualism issue. Tory candidates were successful in three of those contests, and it appeared as if his government's stance had been vindicated. Weir called a general election for June 25, 1969, even though only three years had passed since the previous election.
That turned out to be a strategic error, particularly after the New Democratic Party selected Edward Schreyer as its leader during the campaign. Schreyer was a youthful and charismatic figure from the centrist wing of the NDP, and his party was able to win the support of many centre-left voters, including those who had voted for Pierre Trudeau's federal Liberals the previous year. The NDP won 28 seats against 22 for the Tories. On election night, Weir told his supporters, "The people have spoken. And the people are wrong."[9]
While the NDP came up one seat short of a majority, it was not immediately clear who would form government. Weir initially flirted with supporting a coalition with the Manitoba Liberals in which former Liberal leader Gildas Molgat, despite his party finishing in third place, would have become Premier. However, that came undone when Liberal MLA Laurent Desjardins threw his support to the NDP, which allowed Schreyer to replace Weir as Premier shortly thereafter.
Weir stepped down as PC leader in February 1971, and retired as an MLA in September of the same year. He did not re-enter political life again. Weir died in the evening of April 17, 1985 of a suspected heart attack at his home in Minnedosa, Manitoba.[10]