Crombie was born to Polish/Ukrainian-Canadian immigrants in Toronto, Ontario. She graduated from St. Michael’s College at the University of Toronto. She received her M.B.A. from York University in 1992.

Before entering politics Crombie was an entrepreneur and public affairs consultant that worked with many clients including the Insurance Board of Canada, McDonald’s, and Disney. Crombie and her husband Brian live in Mississauga where they raised three children.

Crombie was elected as MP for the riding of Mississauga—Streetsville as a Liberal Party candidate in the 2008 Canadian federal election, defeating incumbent MP Wajid Khan who had previously crossed the floor from the Liberals to the Conservative Party. After being elected, Crombie served in Stéphane Dion’s Liberal caucus as co-chair of outreach along with Justin Trudeau. Crombie also served as the Liberal Party critic for Crown corporations. She was an active supporter of Michael Ignatieff in his bid for the Liberal leadership. She was later defeated by Conservative candidate Brad Butt in the 2011 federal election.

On September 19, 2011, Crombie was elected to Mississauga City Council in the by-election to succeed Eve Adams as Councillor for Ward 5, winning by slightly more than 200 votes over Carolyn Parrish.

On October 12, 2014, retiring long-serving Mayor of Mississauga Hazel McCallion endorsed Crombie to replace her as Mayor. Crombie later de-feated former city councillor and federal cabinet minister Steve Mahoney to win the 2014 mayoral election.