A member of the Minnesota Democratic–Farmer–Labor Party (DFL), he represented District 67A, which includes all or portions of the Saint Paul Greater East Side, Payne-Phalen, Hayden Heights, Frost Lake, Hillcrest and Phalen Village neighborhoods in Ramsey County, which is in the Twin Cities metropolitan area.

He was raised on the East Side of Saint Paul, graduating in 1971 from Harding High School.

[3] Mahoney was first elected to the House in 1998, when incumbent Rep. Jim Farrell ran for Ramsey County Attorney.

He defeated fellow DFLers Michele Ford, a legislative staffer, and Brian McMahon, a housing specialist, in the 1998 DFL primary election; he went on to defeat Republican Julie Glanz and Reform candidate Andy Lamotte in the general election.

He chaired the Finance Subcommittee for the Higher Education and Workforce Development Finance and Policy Division and Bioscience and Workforce Development Policy and Oversight Division during the 2009–2010 biennium, and the Biosciences and Emerging Technology Committee during the 2007–2008 biennium.