Bonnie Crombie, 54, a former MP, filed papers to register as a candidate Tuesday at Mississauga city hall.
She's the fourth candidate to register so far, with the most prominent being Steve Mahoney, who's also a former MP, a former member of the provincial legislature and a former city councillor.
Crombie was the Mississauga-Streetsville MP from 2008 to 2011 and became a city councillor in a 2011 byelection after losing her federal seat.
She says finding funding for infrastructure and solving gridlock will be among the issues she plans to campaign on.
McCallion, 93, who has been Mississauga's mayor since 1978, has said she won’t be running for re-election this fall.