Senator Flanagan began working in the Massachusetts House of Representatives in 1995 as a Legislative Aide, then as Chief of Staff on four legislative committees. She was elected to the House of Representatives for two terms, representing the 4th Worcester District. She was elected to the Massachusetts State Senate in September of 2008 and was sworn in the first week of January 2009. Senator Flanagan now represents Ashby, Bolton, Precincts 1 &2 of Clinton, Fitchburg, Gardner, Lancaster, Leominster, Lunenburg, Sterling, Townsend and Westminster.

She is currently serving as Assistant Vice-Chairman of the Senate Committee on Ways and Means; Chairman of the Senate Committee on Steering and Policy; Vice-Chairman of the Joint Committee on Transportation; Vice-Chairman of the Joint Committee on Public Service and is a member of the Joint Committee on Telecommunications, Utilities and Energy.

Senator Flanagan has also been appointed to the following commissions at State House: The Autism Commission, the Interagency Council on Substance Abuse, the Commission on Postpartum Depression, the special Commission to Study Electronic Benefit Transfers, the Gaming Policy Advisory Commission the Department of Mental Health Task Force on Staff and Client Safety, Governor Patrick's Children, Youth and Families Advisory Committee and Treasurer Grossman's Online Products Taskforce. She is also a member of the Regional Transportation Authority Caucus, the Gateway Cities Caucus, the Non-Profit Caucus and the Foster Care Caucus.

Senator Flanagan serves on the following boards in the community: the Spanish American Center, the Montachusett Addiction Council, Community Health Connections Family Health Center, the SHINE Advisory Board and the Leominster Education Foundation. She is an honorary board member of the Gardner Community Action Council and Leominster's Allencrest Tutoring Center.