[3] Brennan is a licensed clinical social worker and worked as a policy associate at the Muskie School of Public Service at the University of Southern Maine, where he directed projects related to child welfare, substance abuse, mental health, and public policy.

A special election was held in March 2002, and he ran in the vacant 27th District and won with a plurality of 41%, defeating Republican Sally Vamvakias by just 10 votes.

The meeting featured a 4+ hour discussion on whether to allow OccupyMaine, which was part of the Occupy movement, to obtain a 6 month permit to use two-thirds of nearby Lincoln Park.

[14] Upon the passage of Maine's same-sex marriage law, Mayor Brennan stated that he would look into the possibility of opening Portland City Hall at 12:01 AM on December 29, 2012, the day the law takes effect, to issue marriage licenses in order to have the first legal same-sex wedding in Maine occur in Portland.

[15] In 2013, Brennan was a vocal supporter of a plan to sell two-thirds of Congress Square Park to Rockbridge Capital.

[18] Mayor Brennan announced on June 3, 2015 that he would seek reelection but was defeated by old rival Ethan Strimling.