Hayden Mullins scored on his 550th league appearance to seal Birmingham's Championship status and hand Leeds boss Brian McDermott his first loss.
The 34-year-old midfielder converted in the 71st minute after Paddy Kenny parried Nathan Redmond's effort.
Ravel Morrison hit the crossbar in the first half with Ross McCormack's free-kick into the penalty area clipping the woodwork for the visitors.
Birmingham's win takes them up to 10th in the Championship table.
The goal for Hayden Mullins was his second of the season, coming after he scored in the 2-2 draw at Nottingham Forest on 15 September
Leeds, also mathematically safe in a congested division, drop into the bottom half having won both of their fixtures since the appointment of McDermott earlier this month.
The hosts were the stronger side in the first half as Morrison, on loan from West Ham, pulled the strings.
He almost broke the deadlock with a superb curling effort which came back off the crossbar, while Shane Ferguson fired narrowly wide from just outside the area.
Leeds were denied midway through the half when Jack Butland brilliantly tipped over Michael Tonge's 20-yard strike.
Mullins then failed to find the target from eight yards out before McCormack's free-kick preceded a better spell for the visitors.
Birmingham struggled to find the same rhythm as they did in the first half but, against the run of play, Mullins tucked in the rebound after Redmond's long-range shot was saved.
Birmingham manager Lee Clark:
"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 [Hayden Mullins] 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."
Leeds boss Brian McDermott:
"I thought we should have got something out of the game. When it was 0-0 we were on top and then they scored from nothing.
"When I arrived at the club I put my reputation on the line and to make sure Leeds remained in this division. So far as I am concerned it is job done.
"Everyone must be going in the same direction. If that occurs in this club you have a major chance."
Live Text Commentary
Full Time The final whistle is blown by the referee.89:59
Effort on goal by Samuel Byram from long range goes harmlessly over the target.85:27
Nikola Zigic is flagged offside by the assistant referee. Free kick taken by Patrick Kenny.84:59
Mitch Hancox concedes a free kick for a foul on Lee Peltier. Lee Peltier takes the direct free kick.83:55
Effort on goal by Christopher Burke from outside the box goes harmlessly over the bar.82:43 Substitution
Substitution Adam Drury is brought on as a substitute for Rodolph Austin.81:30
Christopher Burke takes the outswinging corner, Curtis Davies takes a shot. Patrick Kenny makes a save.79:22
Long distance shot from Christopher Burke misses to the left of the goal.78:50
Free kick awarded for a foul by Rodolph Austin on Nathan Redmond. Wade Elliott restarts play with the free kick.77:44
Mitch Hancox crosses the ball, Stephen Warnock makes a clearance.76:43
Ross McCormack gives away a free kick for an unfair challenge on Paul Caddis. Paul Caddis restarts play with the free kick.76:18
Foul by Wade Elliott on Paul Green, free kick awarded. Direct free kick taken by Tom Lees.75:02
Inswinging corner taken right-footed by Ross McCormack.74:45
Rodolph Austin takes a shot. Save by Jack Butland.73:49 Substitution
Substitution Steve Morison is brought on as a substitute for El-Hadji Diouf.71:00 Goal scored
Goal! - Hayden Mullins - Birmingham 1 - 0 Leeds Hayden Mullins scores a goal from inside the six-yard box to the bottom left corner of the goal. Birmingham 1-0 Leeds.70:36
Nathan Redmond takes a shot. Save by Patrick Kenny.65:57
El-Hadji Diouf concedes a free kick for a foul on Paul Robinson. Free kick taken by Jack Butland.60:55 Substitution
Substitution (Birmingham) makes a substitution, with Christopher Burke coming on for Shane Ferguson.60:55 Substitution
Substitution Lee Peltier replaces Michael Tonge.
Inswinging corner taken right-footed by Wade Elliott.59:42
Tom Lees challenges Nikola Zigic unfairly and gives away a free kick. Wade Elliott restarts play with the free kick.59:06
Luke Varney is ruled offside. Jack Butland takes the indirect free kick.58:40
Samuel Byram produces a cross.57:04
Free kick awarded for an unfair challenge on Michael Tonge by Nikola Zigic. Free kick taken by Tom Lees.53:43
Ross McCormack produces a right-footed shot from deep inside the six-yard box which goes wide of the right-hand upright.53:07
Inswinging corner taken from the left by-line by Michael Tonge, Nikola Zigic makes a clearance.52:56
Ross McCormack takes a shot. Blocked by Wade Elliott.49:20
Foul by Paul Caddis on El-Hadji Diouf, free kick awarded. Ross McCormack restarts play with the free kick.47:43
Corner from the left by-line taken by Michael Tonge, save by Jack Butland.47:37
Centre by Ross McCormack, Curtis Davies makes a clearance.46:07
Nikola Zigic produces a left-footed shot from just outside the penalty box that goes wide left of the target.45:33
The assistant referee flags for offside against El-Hadji Diouf. Jack Butland takes the direct free kick.45:01
The referee gets the second half started.45:00 +1:04 Half time
Half Time The ref blows to end the first period.44:02
Free kick awarded for an unfair challenge on Jack Butland by Luke Varney. Free kick taken by Jack Butland.43:25
Free kick awarded for a foul by Hayden Mullins on Michael Tonge. Direct free kick taken by Stephen Warnock.42:17
Nikola Zigic has a headed effort at goal from close range which goes wide left of the target.39:44
Ross McCormack takes a shot. Save made by Jack Butland.37:48
Free kick awarded for an unfair challenge on Paul Caddis by Stephen Warnock. Shane Ferguson crosses the ball from the free kick left-footed from right wing.36:47
Effort from a long way out by Ravel Morrison goes wide right of the goal.34:19
The referee blows for offside against Nikola Zigic. Patrick Kenny takes the free kick.34:12
Centre by Paul Caddis, Tom Lees makes a clearance.33:51
The ball is swung over by Rodolph Austin, save by Jack Butland.32:46
Free kick awarded for an unfair challenge on Luke Varney by Paul Caddis. Direct free kick taken by Stephen Warnock.32:14
Unfair challenge on Curtis Davies by Luke Varney results in a free kick. Paul Robinson takes the free kick.31:38
The ball is swung over by El-Hadji Diouf, save by Jack Butland.30:21
Paul Robinson challenges El-Hadji Diouf unfairly and gives away a free kick. Paul Green takes the direct free kick.27:07
Shot from inside the area by Hayden Mullins clears the bar.26:31
Corner taken left-footed by Shane Ferguson.25:55
Corner taken right-footed by Wade Elliott from the left by-line, clearance made by Tom Lees.24:33
Samuel Byram produces a cross, clearance made by Curtis Davies.23:59
Nathan Redmond takes a shot. Comfortable save by Patrick Kenny.23:09
Corner taken left-footed by Stephen Warnock from the right by-line, Jack Butland makes a save.21:53
Stephen Warnock takes the inswinging corner, Hayden Mullins makes a clearance.21:37
Michael Tonge takes a shot. Jack Butland makes a save.20:36
Foul by Rodolph Austin on Ravel Morrison, free kick awarded. Paul Robinson takes the direct free kick.17:30
Nathan Redmond produces a cross.15:08
Shane Ferguson has an effort at goal and missed to the left of the net.12:56
Rodolph Austin takes a shot. Save made by Jack Butland.12:47
The ball is sent over by Stephen Warnock, Curtis Davies manages to make a clearance.11:48
The ball is sent over by Shane Ferguson, Ravel Morrison takes a shot. Patrick Kenny makes a save.8:10
Effort on goal by Ravel Morrison from long range goes harmlessly over the target.2:54
Foul by Luke Varney on Paul Robinson, free kick awarded. Direct free kick taken by Jack Butland.0:00
The referee gets the match started.
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