Romeo Crennel has coached in some capacity in the NFL since 1981 when he made the jump from coaching defensive linemen at Georgia Tech to coaching special teams and taking on a defensive assistant role with the New York Giants. From that point on he remained in the NFL. He stayed in New York until moving to Cleveland to be the defensive coordinator on a 3-13 Browns team, and then took on the same role with the Patriots, running the defense during the first part of their dynasty.
After that he left to be a head coach. Taking a job in Cleveland, and eventually taking another job in Kansas City.