Clark had every reason to be pleased with his revitalised Birmingham side as they moved into 10th place - their highest since last September.
It was only Birmingham's third home victory of the year and they were indebted to Mullins, who scored his second goal of the season on his return to the side after missing the last 19 games.
Clark paid credit to the 34-year-old who made his 550th league appearance. The Blues manager said: "I plumped for experience on this occasion.
"I thought that the way Leeds were playing with a new manager I just wanted an older head to look after the likes of (Ross) McCormack and he did it well.
"I didn't include Hayden to be the match winner but it was a good occasion to celebrate his 550th game.
"It was a terrific win so far as I am concerned I'm delighted with the three points and a clean sheet.
"We played really well in the first half although we became a little sloppy at the start of the second period. But we then got the goal and kicked on from there and in the last 10 minutes we had further chances to kill off the game.
"The youngsters in the side have given me a huge boost but the frustrating thing for me is that if we had not messed around in the early part of the season and got three positive results they could have taken us into the play-off spots.
"Ultimately I had to give the players who did so well last season an opportunity to repeat the feat. For whatever reason it could not be repeated and I had to go down the route of using the younger players just before Christmas."