She was first elected to Toronto City Council in 1972, and served until 1985 when she ran against incumbent Mayor Art Eggleton, but was defeated.
That year, she was elected to the new Toronto City Council and served until 2003, when she was defeated by Karen Stintz.
Johnston attempted to unseat Art Eggleton as mayor in the 1985 municipal election but was defeated 92,994 votes to 59,817.
In the late 1990s, Johnston's queries into the city's computer leasing deal led to the establishment of the MFP Inquiry which unearthed corrupt practices in the deal and reforms in the city's public accountability framework.
In her final term she supported a controversial condominium development at the corner of Yonge and Eglinton in 2002 adjacent to her ward.