Born and raised in Collingwood, Ontario, she is the daughter of former Collingwood mayor Ron Emo. McMahon has a bachelor's degree in French literature from Carleton University in Ottawa. Since 1989, McMahon has resided in the Beaches - East York area. She describes herself as a "fiscally conservative environmentalist."
McMahon is former teacher of English as Second Language and later a community activist. McMahon worked at the environmental organization Live Green Toronto, founded the East Lynn Park Farmers' Market and is a member of Danforth East Community Association.