 Costello was elected to the Newburyport City Council as a councilor-at-large in 1997 and, a year later, became field director for Congressman John F. Tierney's (D-Salem) campaign.Following Congressman Tierney's successful election, Costello worked as the chief of staff for State Senator Joan Menard (D-Somerset), former chair of the Massachusetts Democratic Party.Costello was elected to the House of Representatives in November 2002.