She was reelected in 1986 in the newly created riding of Surrey-Newton and became a cabinetminister under Premier Bill Vander Zalm, serving in various portfolios. She had previously served under Vander Zalm when she was a councilor and he was the mayor of Surrey.
Vander Zalm appointed Johnston deputy premier in 1990. When Vander Zalm resigned, on April 2, 1991, caucus selected her as interim leader over attorney generalRussell Fraser by a vote of 21–17. As such, she was appointed premier on April 2, 1991, making her Canada's first female first minister.[2]
Johnston waited as long as she could, finally calling an election for October. However, her long association with the scandal-plagued Vander Zalm hampered her chances of winning in her own right. Her party was also bitterly divided because of the leadership contest, and she had little time to repair the breach before the writs were dropped.
The Socreds were heavily defeated by New Democratic Party, led by Mike Harcourt. Moreover, many moderate Socreds switched their support to the previously moribund BC Liberals. The Socreds lost more than half of their popular vote from 1986 and were cut down to seven seats, falling to third place in the Legislative Assembly behind the NDP and Liberals.
Johnston herself lost her own seat to the NDP's Penny Priddy by over 10 points, and all but seven members of her cabinet were defeated. Harcourt later said that he preferred facing Johnston rather than McCarthy, believing McCarthy would have been a tougher opponent in a general election.
Johnston resigned as leader of the Social Credit Party on January 11, 1992 and was replaced by McCarthy. After her defeat, Johnston retired from politics and has had a low public profile.