Brentwood is a town in Rockingham County, New Hampshire, United States. At the 2020 census, its population was 4,490. Brentwood has been the county seat of Rockingham County since 1997. An Abenaki tribe called the Pennacook farmed, fished and hunted in what is now Brentwood. Two main foot trails ran through the town, one along the Exeter River, wh…Brentwood is a town in Rockingham County, New Hampshire, United States. At the 2020 census, its population was 4,490. Brentwood has been the county seat of Rockingham County since 1997. An Abenaki tribe called the Pennacook farmed, fished and hunted in what is now Brentwood. Two main foot trails ran through the town, one along the Exeter River, where arrowheads and other stone and wooden artifacts have been found. At Pickpocket Dam, this pathway joined with the Pentucket Trail leading south to Pentucket and north further into Pennacook territory.