She was formerly a Member of the Scottish Parliament (MSP) for the North East Scotland region for 70 days in early 2016 and a councillor in Dundee from 2012 to 2016.

In October 2013, she was selected by Scottish Labour to contest Dundee East at the 2015 United Kingdom general election.

[2][3] When Richard Baker resigned from the Scottish Parliament in January 2016, Labour nominated Brennan as his replacement as she had been next on their North East Scotland regional list.

She stood at the 2016 Scottish Parliament election where Labour had placed her third on their regional list for North East Scotland, but she was not re-elected.

[3] A member of Campaign for Socialism, she was appointed in November 2017 by Scottish Labour leader Richard Leonard to serve as his chief of staff.