After a thirty-year career in drug law enforcement, Rex McMillan retired in 2000 as a Senior Special Agent with the United States Customs Service. His career began with the Customs Service in 1970, but McMillan transferred to the newly created Drug Enforcement Administration in 1973, where he served in a supervisory capacity. McMillan returned to the Customs Service in 1987 where he remained until his retirement.

McMillan's entire career was devoted to stopping the flow of illegal drugs into the United States. During his career with Customs and DEA, McMillan designed instructional programs for various phases of drug law enforcement, including air and marine smuggling, informant development and management, case development, and surveillance. He has taught officers from local, state, and federal agencies in various drug interdiction courses, including surveillance tactics.