Mullen attended West Virginia University after high school, where was a member of the Mountaineers football team from 1981 to 1984. After his freshman season, Mullen considered a transfer, but decided to stick it out after speaking with Risha. Mullen spent two years as a reserve wide receiver for the Mountaineers, but he continued to work hard to earn more playing time. Mullen earned playing time in 11 games during the 1983 season, amassing 19 receptions for 343 yards and three touchdowns. Mullen was also utilized as a kickoff returner, returning 12 kicks for 237 yards. Mullen's senior year at West Virginia was his most productive statistically, as he caught 31 passes for 557 yards and three touchdowns. In total, Mullen amassed 80 receptions for 1,332 yards and six touchdowns during his collegiate career. Gary was also part of three Mountaineer bowl game victories: the Peach Bowl in 1981, the Hall of Fame Classic Bowl in 1983, and the Bluebonnet Bowl in 1984.
Professional career
After not hearing his name called during the 1985 NFL Draft, Mullen was signed by the Detroit Lions, but was released before the season began. In the summer of 1986, Mullen signed with the Pittsburgh Steelers, but he was unable to shake a bad case of the flu; Mullen left the Steelers by his own will. Mullen made one final attempt to continue his football career, when he attended a tryout for the upstart Arena Football League. Going into the tryout, Mullen wanted to join the Pittsburgh Gladiators, as they were close to where he grew up.
Denver Dynamite
Just one day before Mullen was ready to quit on his football dream and join the U.S. Army, he received a phone call inviting him to try out to play for the Denver Dynamite of the Arena Football League. Mullen became a focal point for the Dynamite offense, but with the Arena Football League then making players play both ways, he was also a key player on defense. Mullen didn't take the lack of a defensive background as an excuse to not play hard on defense. Mullen had an outstanding season for the Dynamite, leading the team in receptions, receiving yards and receiving touchdowns. The Dynamite tied for the best record in the AFL, which earned them a spot in ArenaBowl I. Mullen won ArenaBowl I MVP during his team's 45–16 victory over the Pittsburgh Gladiators.
Chicago Bears
Mullen's play during the 1987 season earned him a chance to play for the Chicago Bears during the 1987 NFL strike. He played in all three games that the strike lasted, making two career receptions for 33 yards.
Los Angeles Cobras
After the Dynamite suspended operations during the 1988 season, Mullen went to play for the expansion Los Angeles Cobras as they owned the rights to all Dynamite players. Mullen's one year of experience was heavily leaned on by the Cobras, and Mullen enjoyed being the main target for the Cobras. Mullen lead the Cobras with 61 receptions for 823-yards and 17 touchdowns. The Cobras finished in 4th place, which was good enough to earn them the 4th seed in the playoffs. They were matched against the 1 seed, Chicago Bruisers, with a chance to make it to ArenaBowl II. The Cobras lost 16–29, while Mullen paced the offense with 7 receptions for 78-yards and 1 touchdown. After the season ended, the Cobras, as well as other teams, folded and the league playing again in 1989 looked in doubt. When they league announced that they would be playing in May, Mullen said that he would not play in 1989 due to all the pay cuts he would be receiving.
Detroit Drive
After sitting out the 1989 regular season to get himself more established at his full-time job, Mullen signed with the Detroit Drive during their playoff run. The Drive won ArenaBowl III. Mullen returned to the Drive in 1991, and teamed with fellow WR/DB George LaFrance and QB Art Schlichter, the Drive's offense stood as one of the most challenging to defend. He helped the Drive to a three-peat with a 37-yard touchdown reception, when the Drive defeated the Dallas Texans 51-27 in ArenaBowl IV. In 1992, the Drive were again on their way to the ArenaBowl, this time they faced the Orlando Predators.
Cincinnati Rockers
Mullen was acquired by the Cincinnati Rockers for future considerations in 1993.
Milwaukee Mustangs
When the Milwaukee Mustangs were formed in 1994, Mullen was selected in their expansion draft. Mullen signed a contract with the Mustangs a few days later.
Personal
Both of Gary's brothers played professional football. His younger brother Keith Mullen, played arena football in 1991 with the Columbus Thunderbolts, while his older brother Davlin Mullen played for the New York Jets.
