Brown has said the rescue efforts of Army National Guard during the Northeastern United States blizzard of 1978 impressed him. When he was 19, he joined the Massachusetts Army National Guard, received his basic training at Fort Dix, New Jersey, and attended Reserve Officers' Training Corps (ROTC) classes at the campus of Northeastern University. He was trained in infantry, quartermaster, and airborne duties, and in 1994 he joined the Judge Advocate General's Corps (JAG). He was active in the Guard for 35 years rising to the rank of colonel. As the Army National Guard's head defense attorney in New England, Brown defended Guard members who had disciplinary difficulties such as positive drug tests, and provided estate planning and real estate advice to those who were about to deploy to war zones. He spent ten days to two weeks with the Guard in Kazakhstan and a week in Paraguay.