In 1957, he was assigned to a two-year position at CIA headquarters in Langley, Virginia, and subsequently to Pakistan and the Philippines. In 1966 he was appointed Senior Province Advisor in Quảng Trị Province, in the Republic of Vietnam, the only CIA officer to hold such a post, where he served until the summer of 1968. In January, 1968, during the Communist Tet Offensive, Brewer was instrumental in the allied effort to defeat a large scale assault on Quảng Trị City and nearby South Vietnamese military installations by the North Vietnamese People's Army of Vietnam (PAVN). The assault was intended to capture and hold the city and to gain control of the province, and along with the attempt to capture and hold the city of Huế, one province south of Quảng Trị Province, it was one of the North's primary goals in mounting its offensive. Following his service in Vietnam, Brewer spent a tour in Northern Thailand, assisting the Thai government in its efforts to defeat a Communist insurgency, and to eradicate opium production in the Golden Triangle area of Burma (Myanmar), Laos and Thailand. He retired from government service in 1973.