His career best game came against Stanford, when he recorded five receptions for 84 yards, including a 19-yard TD pass from quarterback Jimmy Jones to help the Trojans beat the Indians 26–24.
Selected in the fourth round (86th overall) of the 1971 NFL draft, Mullins mostly played right offensive guard for the Pittsburgh Steelers for his entire professional career (1971–1979), next to All-Pro centers Ray Mansfield (1971–1975) and Mike Webster (1976–1979).
Mullins is remembered for a play in Super Bowl IX when he pulled on a sweep leaving an uncontested path for Franco Harris to trot in the end zone.
Prior to the 1980 season, he was waived during training camp and after declining offers to continue his career with the Cleveland Browns and Houston Oilers, Mullins retired.
Along with other players for the Steelers, Mullins also had a brief film career with a cameo appearance in The Rocky Bleier Story for MTM Enterprises.