[1] Previously, McMahon served on the Toronto City Council from 2010 to 2018, representing Ward 32 Beaches—East York.

She founded the East Lynn Park Farmers market and helped organize an anti-idling campaign at her children's school.

In October 2017 McMahon announced she would not be running for re-election in 2018, confirming a promise made when she was first elected in 2010.

[7] During her time as a councillor she brought forward two motions to Toronto City Council calling for term limits, but both were defeated.

[7] In October 2020, she announced her candidacy for the Ontario Liberal Party nomination in the provincial electoral district of Beaches—East York.