Brentwood is a city in Contra Costa County, California, United States. It is located in the East Bay region of the San Francisco Bay Area. The population is 51,481 as of 2010. Brentwood was originally laid out on land donated from property owned by John Marsh, an East Contra Costa County Pioneer who acquired Rancho Los Meganos, the land Grant that Brentwood is built upon, in 1837 from Jose Noriega. Brentwood was named after Marsh's ancestral home, the town of Brentwood in the County of Essex, England.