The Boro boss sent on new signing Faris Haroun at the break and saw the Belgium international score on his debut to put the home side ahead. Barry Robson had blasted them level with a free-kick after Adam Rooney put the Blues ahead from the penalty spot.
And another summer signing, former French Under-21 international Malaury Martin, snapped up on a free transfer from Blackburn, was also introduced to score his first goal for the club as Boro extended their unbeaten start to the season.
"It is nice to able to bring quality off the bench instead of relying on youngsters," said Mowbray.
"For a lot of games while I've been here it has been a case of putting teenagers on the bench but we are getting to the position now when I can replace quality with quality and that helps."
Martin sealed the victory that took Boro back up to third place in the npower Championship with a sizzling 28-yard strike.
"He has been doing that in training," said Mowbray. "He cuts in and unleashes and a lot of them are unstoppable like the one we saw.
"It was nice for that pair to get off the mark with goals but it was especially pleasing as a team that we worked hard, we didn't rush it and we battled back to take an important three points."
Mowbray was less enthusiastic when discussing a hamstring strain suffered by striker Scott McDonald which could sideline him for weeks. And with no other proven goal-getters apart from Marvin Emnes on the books he suggested he will look for new recruits.
"Scott has picked up an injury and that is bad news for us because he is an important player," he said. "We will have to assess it but it looks like he could miss a few games now which leaves us with a little bit of a problem. But we're working on it."
Birmingham boss Chris Hughton pointed to his own bench as he explained City's sluggish second half display.
"Legs were tired in the second half, possibly because we had a difficult game on Thursday night in the Europa League," he said.
"But also we have lost three important players this week - Liam Ridgewell, Morgaro Gomis and Jordan Mutch - and that has left us with kids on the bench and unable to change it too much.
"In the first half we did well enough but in the second we tired, although we added to our own downfall by giving away a soft goal so early in the second half. There was no need to give away a free-kick there. That gave Boro the momentum.
"Was there a touch of offside about their second goal? Maybe, it was certainly very close. It was a fortuitous rebound for him.
"And we should have had a free-kick for a foul on David Murphy in the build up for the third but these things happen."