Cullen's other credits included roles on Dr. Finlay's Casebook, Redcap, Department S, Blake's 7, Return of the Saint, Sorry! and The Bill. He also had regular roles as Warren Kent on Emergency-Ward 10 (1966-67), Geordie Watson on When the Boat Comes In (1977, 81) and Angus Hart on Family Affairs (1997-99). However, he will be best remembered for playing PC/Det. Con. Skinner in over two hundred episodes of Z-Cars from 1969-75.
Film appearances included the radio operator Steinman in Voyage of the Damned (1978) and Baron Sterling in Dawn of the Dragonslayer (2011).