Flanagan served as a legislative aide and then chief of staff to then-Leominster State Rep. Mary Jane Simmons.

[1] In 2004 Simmons announced she would not seek re-election to the 4th Worcester district due to health concerns,[2] and Flanagan ran for the open seat.

Flanagan won the primary and general election, and served two terms as a Massachusetts State Representative.

Flanagan served as a Massachusetts State Senator for the Worcester and Middlesex district, which includes her hometown of Leominster.

She first won the State Senate seat in 2008, winning a contested Democratic primary and facing no general election opponent.