On 22 October 2018, Jeffrey was narrowly defeated in the mayoral race by former Ontario Progressive Conservative leader Patrick Brown.

Jeffrey joined the Ontario Liberal Party in 2003 after being recruited by Greg Sorbara to run in the provincial election of 2003.

[4][5] Between 2005 and 2010 she served in a variety of positions including Parliamentary Assistant to the Ministers of Children and Youth Services, Democratic Renewal, Intergovernmental Affairs, Citizenship and Immigration and Transportation.

[14] After taking office as mayor, Jeffrey appointed former Premier Davis to a panel tasked with bringing a university to Brampton.

[9][10] Despite that, and her background as a Liberal, Jeffrey held a fundraiser at the Conservative Party-associated venue the Albany Club of Toronto, accepting endorsements from former Ontario Progressive Conservative Party presidents Richard Ciano and Ken Zeise, as well as Michael Diamond, Premier Doug Ford's leadership campaign manager.

[15] According to the Toronto Star, Jeffrey had the backing of "PC party operatives — Doug Ford’s campaign manager organized a fundraiser for her".

[16] Jeffery lost her re-election bid to Patrick Brown by a narrow margin in the 2018 Peel Region municipal elections.