[2] It includes the census-designated places (CDPs) of Marshfield, Marshfield Hills, Ocean Bluff-Brant Rock, and Cedar Crest, and shares the Green Harbor CDP with the town of Duxbury.

Marshfield is bordered by Massachusetts Bay to the east, Duxbury to the south and southeast, Pembroke to the west, Norwell to the northwest, and Scituate to the north and northeast.

The Scituate neighborhoods can be reached by land by two bridges, by boat, or by foot along Rexhame Beach.

Marshfield is the site of several small forests and conservation areas, including the Daniel Webster Wildlife Sanctuary along the Green Harbor River and the North River Wildlife Sanctuary along Route 3A.

The following beaches comprise Marshfield's 5-mile-long (8 km) public seashore: Rexhame, Fieldston, Sunrise, Ocean Bluff, Brant Rock, Blackman's Point, Blue Fish Cove (along Cove Street between the camp ground and the big rock), and Green Harbor (also known as "Burke's Beach").

On the national level, Marshfield is a part of Massachusetts's 9th congressional district, and is currently represented by William Keating (D).

The town is represented by Patrick O'Connor (R) in the Massachusetts Senate as a part of the Plymouth and Norfolk district, which includes the towns of Cohasset, Duxbury, Hingham, Hull, Norwell, Scituate and Weymouth.

[15] The town is patrolled by the First (Norwell) Barracks of Troop D of the Massachusetts State Police.

The current board of selectmen includes a direct descendant of Pilgrim Richard Warren.

The town has its own police and fire stations, with firehouses located in Marshfield Hills, Ocean Bluff and near the fairgrounds.

Route 139 loops through the town, with a long portion passing along the beaches of the Ocean Bluff and Fieldston neighborhoods before heading north and east into Pembroke.

Route 3A also passes through the town, entering from the south in Duxbury and exiting over the North River into Scituate.