Crombie previously served as the sixth mayor of Mississauga from December 1, 2014, until January 12, 2024, and as the member of Parliament (MP) for Mississauga—Streetsville from 2008 to 2011, sitting as a Liberal.

When Bonnie was three her parents separated, and she and her mother relocated to her grandparents' large home in Toronto's High Park neighbourhood.

He was my father.”[6] In 1982, she graduated from St. Michael's College at the University of Toronto with a Bachelor of Arts in political science and international relations.

[16] On September 19, 2011, Crombie secured a seat on Mississauga City Council through a by-election, succeeding Eve Adams as Councillor for Ward 5.

She won by a margin of slightly over 200 votes, defeating Carolyn Parrish and Eve Adams's ex-husband, Peter.

[30][31] During a Liberal leadership debate on The Agenda with Steve Paikin, hosted by TVO on November 15, 2023, Crombie declared that she would not seek re-election in 2026, concluding her tenure as the mayor of Mississauga after three terms.

[32] After winning the 2023 leadership election of the Ontario Liberal Party, Crombie announced on December 13 that she would resign as Mississauga mayor, effective January 12, 2024, after completing the city's and Peel Region's budgets before stepping down.

[48] It was later announced that Crombie would not seek the seat in Milton, but Galen Naidoo Harris, son of former Education Minister Indira Naidoo-Harris, would.

[50] Crombie announced on Monday, March 18, 2024, that should she become premier after the 2025 election, she will refrain from implementing a provincial carbon tax as part of her climate policy,[51][52] diverging from the approach of Trudeau and the federal Liberals.

"[54] 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 Dr. Jane Philpott.

This move is seen by many as another attempt to distance the party and its leader from the unpopular federal Liberals and Prime Minister Justin Trudeau.