Following a largely unremarkable early coaching career serving primarily as a collegiate and USFL assistant, Marcum became the coach of the Denver Dynamite, one of the original AFL franchises, and led them to the championship of the first-ever ArenaBowl, but the team suspended operations after its initial season in 1987. Not waiting for the Dynamite to resume operations (which they later did for three years starting in 1989),
he then went on to coach the Detroit Drive for that team's entire existence save 1990, when he was an assistant with the University of Florida. This team became the AFL's first dynasty, playing in the ArenaBowl during every year of its existence. However, this team likewise folded, but Marcum's success with them became the basis for his hiring and tenure with the most successful Arena team ever, the Tampa Bay Storm, where he achieved his greatest fame, and arguably the greatest fame of any Arena coach (as of 2014), winning four more ArenaBowl championships, including another back-to-back run (1995 and 1996).
On February 17, 2011, Marcum resigned as head coach and general manager of the Tampa Bay Storm after 15 seasons with the team, less than a month before the season was set to begin. His resignation came after it was revealed that in a deposition given in a lawsuit between himself and former team owner Robert Nucci, Marcum had admitted to receiving and forwarding via his work e-mail account material that was pornographic and racially insensitive. Marcum stated that he would not be able to go forward as head coach as the controversy would cause too much of a distraction. It was reported that Marcum may have been fired had he not resigned. Marcum went on to serve as an assistant coach with the New Orleans Voodoo in 2012 and the Storm's fiercest rivals, the Orlando Predators, in 2013.