McMullan made his debut for the Donegal senior football team in 1981, at the age of 18. He helped them to win the 1983 Ulster Senior Football Championship, playing against Cavan in that year's final. He was again part of the provincial championship winning side of 1990, the same year as he was awarded his only All Star.
In 1992, he again won an Ulster Championship medal, and this time, helped his team to win the All Ireland final as well. It was just a few minutes before this match that the manager at the time, Brian McEniff was informed that McMullan's brother, Gerard, had died from cancer. He chose not to inform McMullan or anyone else on the team until after the match. It was later revealed that this had been a miscommunication, and this was not the case. Gerard died a year later.