Provincial politics (2023–2026)
In early 2023, media reports, citing sources within the party, speculated that Crombie was contemplating a run in the 2023 Ontario Liberal Party leadership election scheduled for December. Her active participation was noted during the Ontario Liberal Party's annual general meeting in March 2023 and the Liberal Party of Canada's national convention in May 2023 in Ottawa.
On May 23, 2023, Crombie confirmed the speculation by announcing the formation of an exploratory committee. On June 14, she officially launched her campaign at an event in Mississauga. She took an unpaid leave of absence from her role as mayor of Mississauga starting October 7. However, she returned on November 27 to participate in the city's budget process.
Crombie was considered the front-runner in the Liberal leadership race. She ran on a pledge to make life more affordable, strengthen Ontario's healthcare system, build housing and infrastructure, improving the public education system, and fighting climate change. During the race she referred to herself a "a very centrist person", "very fiscally responsible", and "socially very progressive". She was elected party leader at the December 2, 2023, on the third ballot. She resigned as mayor on January 12, 2024. In August 2026, it was reported that the Progressive Conservative Party had sent a party official to follow Crombie while she vacationed in Jamaica shortly after her leadership victory.
Following the resignation of Parm Gill from the Ontario Legislature in January 2024, Crombie stated that she was considering running in the resulting Milton by-election, but later decided not to seek the seat.
Crombie announced on March 18, 2024, that if she became premier after the 2025 election, she would refrain from implementing a provincial carbon tax as part of her climate policy, diverging from the approach of Prime Minister Justin Trudeau and the federal Liberals. The next day, she declined to comment on a proposed federal carbon tax supported by the Liberal Party of Canada, saying "I'm not here to tell the federal government how to do their job."
On August 21, 2024, the Ontario Liberal Party announced their keynote speakers for their annual general meeting that included former British Columbia Premier Christy Clark and former Canadian Health Minister Jane Philpott. This move was seen by many as another attempt to distance the party and its leader from Trudeau and the federal Liberals.
On September 21, 2024, Crombie and the Ontario Liberal Party unveiled their new logo alongside their slogan, "More for You".
On December 3, 2024, Crombie criticized the federal carbon tax, describing it as "wrong."
Crombie led the Liberals to a third-place finish in the 2025 Ontario general election. While the party picked up enough seats to regain official party status for the first time since 2018, the gains were not enough to displace the Ontario NDP as official opposition. Crombie also failed to win a seat in the Ontario legislature, placing second in Mississauga East—Cooksville. Nevertheless, she vowed to continue as Liberal leader. On March 1, 2025, the Ontario Liberal Party's executive council voted unanimously in support of Bonnie Crombie remaining party leader.
Crombie received 57% support in a leadership review vote during the party's annual general meeting. This was above the 50% support required by the party's constitution, but short of the 66% many in the party had been calling for. Crombie initially announced that she would be staying on as leader, but hours after the results were announced issued a statement that she planned to resign upon the selection of a new leader. She formally resigned as leader on January 14, 2026, with a successor to be chosen at a later date.
