Johnston was first elected in 2014 as the mayor of the
City of Glenorchy, winning 59% of the vote against incumbent mayor Stuart Slade. In 2015, Johnston claimed that the city council had breached regulations by voting on making 16 employees redundant without her presence. Federal independent MP for the
Division of Denison,
Andrew Wilkie, who endorsed Johnston for mayor, criticised this as "puerile behaviour", saying that some aldermen were "sore losers" and trying to create an impression of chaos under Johnston's mayoralty. She was re-elected in 2018 as mayor, winning 86.4% of the vote. In a press release on 27 February 2021, Johnston announced she would run for the seat of
Clark in the next Tasmanian state election.
A snap election was ultimately called a month later, with the election date set for 1 May 2021. After the counting of final preferences, Johnston won the final seat in Clark with 11% of the primary vote. Johnston's win was the first time an independent candidate had won a seat in the House of Assembly since
Bruce Goodluck in 1996. In March 2022 Johnston said of the
Tasmanian Integrity Commission, that there is "snowball's chance in hell that this government will move to reform and strengthen the Integrity Commission in Tasmania". Johnson was re-elected in the
2024 Tasmanian state election. Johnston provided the
Rockliff Liberal Government with
confidence and supply alongside all three members of the
Jacqui Lambie Network and former-Labor leader and independent MP for
Franklin,
David O'Byrne. She was re-elected at the
2025 Tasmanian state election. ==References==