As an undergraduate, Costello organized and led a student program for three years that built housing for some of the poorest Americans living in Appalachia.

He prosecuted hundreds of cases involving drug possession and distribution, domestic violence, assault, battery, robbery, and gang activity.

Following 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.

He served as the Chairman of the Joint Committee on Public Safety and Homeland Security and is a member of the Democratic leadership team in the House.

Costello was one of the sponsors of identity theft legislation signed into law by Governor Deval Patrick.