Fullerton entered provincial politics in 2016, declaring her intention to run against then-Progressive Conservative MPP Jack MacLaren in Kanata-Carleton.

On May 28, 2017, MacLaren was kicked out of the Ontario PC caucus and barred from being a candidate in the 2018 election,[6] leading to a two-way race between Fullerton and Police Sergeant Rick Keindel.

[10] During the COVID-19 pandemic in Ontario, about 3,800 residents died from COVID and thousands more infected, by the time Fullerton was shuffled out of that role in June 2021.

[10] In April 2021, both the Auditor General of Ontario and the independent Long-Term Care COVID-19 Commission issued reports finding that the Ontario government had reacted too slowly to the spread of COVID-19 and that decades of neglect from Fullerton and her predecessors had left it systematically unprepared to deal with a pandemic.

Fullerton came under fire for dodging questions on the reports and blaming the actions of Liberal predecessors, but committed the government to implementing some of the recommendations.

During her tenure, Ontario failed to meet its targets of 8,000 families getting funding for autism therapies by Fall 2022 and stopped updating the public and the press on progress on the file.

[10] On March 24, 2023, Fullerton issued a public statement that she was resigning as both a cabinet minister and MPP, effective immediately.