Two main foot trails ran through the town, one along the Exeter River, where arrowheads and other stone and wooden artifacts have been found.

Beginning in 1738, residents living in the southwestern portion of Exeter, now Brentwood and Fremont, petitioned to be set off, but were denied.

[5] Eighteen people wrote in dissent, saying that even the proposed new town would require travel more than two miles to a new meetinghouse.

[6]: 4  On June 26, 1742, colonial governor Benning Wentworth set Brentwood off from Exeter and incorporated it.

[7] The meetinghouse was planned at "the west side of the 'Gully'", but those living south of the Exeter River said it was hard to reach in spring and fall.

Wentworth issued a King's Patent to establish a new town called "Keeneborough Parish", named after his friend, Sir Benjamin Keene (1697–1757), English minister to Spain.

[3] Retail development is focused along Calef Road, though home businesses exist elsewhere for historical reasons.

Brentwood had to reimburse Exeter, but only the amounts for upgrading Pine Road to handle the heavy vehicles.

[10] The 2014 fatal shooting of police officer Stephen Arkell, who arrived at a condominium to investigate a domestic disturbance,[11] led to many memorials and continuing events in his name, such as an annual 5K run.

In 2021, the section of Route 125 within Brentwood was officially named the Officer Stephen Arkell Memorial Highway.

[12][13] Brentwood is a nearly square quadrangle, except that the town line follows the Exeter River in the southeastern corner.

[1] The highest point in Brentwood is Great Hill, at 275 feet (84 m) above sea level, whose summit is in the southeastern corner of the town.

[14] Brentwood's executive is a five-person board of selectmen, serving staggered three-year terms and meeting weekly (biweekly in spring and summer).

Citizen petitions have sought to change Brentwood's two town meetings to SB 2, but have always fallen short of the 60% vote required.

[18] At the park and ride lot off Calef Road just north of Brentwood, there is bus service to Boston, Massachusetts and its Logan Airport.

[21] Grace Ministries holds religious services and a Christian grade school in a former industrial building on Calef Road.