It is represented by Democrat Jake Auchincloss.

In prior years, the district stretched from Brookline to Fitchburg.

The shape of the district underwent some changes effective from the elections of 2012, after Massachusetts congressional redistricting to reflect the 2010 census.

[3] Most of Plymouth County and the South Coast are included in the new 9th district.

For a very brief time (1793–95) it represented part of the District of Maine.