In November 2012, Brennan was first elected to the SMC Harbor Commission. Brennan received 120,656 votes from a field of six candidates. She beat two longtime incumbents and received over 25,000 more votes in a landslide victory.

In November 2016, Commissioner Brennan was re-elected to the San Mateo County Harbor District Board of Commissioners. Brennan received 156,292 votes from a field of five candidates. She received the highest number of votes for a 4-year seat.

In 2015, she advocated for the inclusion of women in the Mavericks big wave surf competition, she persuaded the California Coastal Commission to require that women athletes be included. Building on her success at Mavericks, she co-founded the Committee for Equity in Women’s Surfing and led the charge for global equal pay in professional surfing. She’s made numerous remarks and given presentations at Coastal Commission hearing and State Lands Commission meetings. In 2018, she successfully advocated for gender pay parity. Her work is the subject of two documentary films, hundreds of news stories including the the New York Times Magazine feature published Sunday, February 10, 2019, “The Fight for Gender Equality in One of the Most Dangerous Sports on Earth.”

She serves as an elected San Mateo County Democratic Central Committee member and delegate to the California Democratic Party and is the founder of the Coastside Progressive Democrats.