[4] According to the United States Census Bureau, the town has a total area of 51.4 mi2 (133.1 km2), of which 51.3 mi2 (133.0 km2) is land and 0.1 mi2 (0.1 km2) (0.10%) is water.

Underhill is home to the highest summit within the state, Mount Mansfield, which has a peak elevation of 4,393 feet (1,339 m) above sea level.

The Browns River is the primary waterway within the town and originates as a stream from Mount Mansfield.

It runs southwest and converges in the valley, just east of the village of Underhill Center with two other streams that also originate from Mount Mansfield; first with Stevensville Brook, and then with Clay Brook about 0.5 miles (0.80 km) further.

Underhill Center is an unincorporated village (i.e. not a legally defined entity) located around the intersection of River Road and Pleasant Valley Road, on the eastern side of the town closer to the foot of Mount Mansfield.

Located within the village area are the Town Hall and municipal offices, the town hall park, a recreation area (with tennis courts, half-court for basketball, and a small swimming pond), Casey's Hill (a municipally owned sledding hill), and the St. Thomas Catholic Church.

On November 4, 2014, the communities of Bolton, Jericho, Richmond, Underhill ID and Underhill Town voted to form the Mount Mansfield Modified Union School District also known as the MMMUSD.