Mullin, Wrexham’s top scorer for the past three seasons, came off the bench 63 minutes into the much-anticipated League One game at St Andrew’s.
The incident in the 81st minute preceded scuffles between both sets of players that led to Mullin and the hosts’ Krystian Bielik being among those booked.
Bielik was then dismissed after collecting a second yellow card five minutes later for a challenge on Wrexham’s Andy Cannon.
Birmingham manager Chris Davies said he had missed the initial altercation.
“I had my back turned to the melee because there was all that nonsense going on,” he said.
Wrexham captain James McClean was also shown a yellow card after his part in the clashes between players. He was later substituted and appeared to catch an object thrown from the stand as he walked around the perimeter of the pitch.
Birmingham came from behind to win the fixture and end Wrexham’s unbeaten start to the season, with the hosts moving into second behind Phil Parkinson’s side on goal difference.