During the Civil War, Brewer was a state military agent with the rank of major. He served as member of the board of supervisors of Chautauqua County, New York from 1868 to 1879, and was a delegate to the 1872 Republican National Convention. He served a member of the New York State Assembly (Chautauqua County, 1st District) in 1873 and 1874. He was the government director of the Union Pacific Railroad for four years under Presidents Grant and Hayes. He was appointed manager of the state insane asylum, Buffalo, New York, in 1881.