O'Brien was invited by McGinn to the Seattle chapter of the Sierra Club as a volunteer treasurer, despite his previous lack of interest in politics. O'Brien became politically involved with the group and was elected as political chair of the Seattle chapter and chair of the Washington state chapter. During the 2007 election, O'Brien served as the group's spokesperson in the debate over the Roads and Transit proposition, which would have bundled Sound Transit projects with road expansion and was opposed by the Sierra Club. The ballot measure was defeated and a transit-only version, Sound Transit 2, was proposed for the following year with the support of the Sierra Club. Mike O'Brien was a representative of the Sierra Club on the Alaskan Way Viaduct Replacement Project Stakeholder Advisory Committee in 2008. During the 2008 campaign, O'Brien left Stokes Lawrence to be a full-time volunteer for the Sierra Club and was encouraged to run for office by McGinn and others in the environmental community.
Political career
2009 election
In February 2009, O'Brien announced his candidacy for a city council seat that incumbent councilmember Richard McIver planned to not run for. During the August primary for Position 8, O'Brien advanced with 35 percent of votes and was set to run against second-place finisher Robert Rosencrantz. O'Brien won the general election with 58 percent of votes and was sworn-in on January 1, 2010, alongside newly-elected mayor Mike McGinn.
2015 election
O'Brien ran for the newly created District 6 position for the Seattle City Council, primarily covering Ballard, Fremont and Green Lake. He won the primary and defeated Catherine Weatbrook in the November general election. During his term as the District 6 councilmember, O'Brien attracted criticism for his positions on transportation, housing, homelessness, policing, and taxes. At an after-party event following a gala for the Nordic Museum on May 5, 2018, O'Brien was reportedly asked to leave and subsequently forcibly removed by members of the Pacific Fishermen Shipyard. O'Brien has attracted criticism from the group for his position on the extension of the Burke-Gilman Trail in Ballard. Facing unfavorable polling results and opposition from groups like the Metropolitan Chamber of Commerce, in February 2019 O'Brien announced he would not seek reelection to the Seattle City Council.