He first came to prominence on the club scene as a member of the senior team in the early 1970s. In 1972, Mullins lined out in his first county championship decider. College side UCD provided the opposition on that occasion and a close game developed. Key goals by St Vincent's proved the difference as Mullins's side won by 2–8 to 0–9. It was his first county senior championship winners' medal. St Vincent's subsequently represented the county in the provincial club series of games and even reached the final. Westmeath champions the Downs were the opponents. The game turned into a complete rout as Mullins's side took complete control. A 6–10 to 2–5 score line gave St Vincent's a first Leinster club title. An All-Ireland final against Cork champions Nemo Rangers followed for Mullins. The sides were level six times in that game and a pointed free by Jimmy Keaveney secured a draw. The replay saw St. Vincent's being completely outclassed as Nemo won by 4–6 to 0–10.