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.
Brentwood's first post office was established in 1878. The city incorporated in 1948.