The England international fired the home side into a fifth-minutelead and then added a second six minutes later to put them firmly inthe driving seat.
Bent at the double: Sunderland's top scorer hails his second of the afternoon
However, Birmingham emerged after the break in a much moredetermined mood and having reduced the deficit through Cameron Jerome'sstrike on the hour they threatened to leave with a point.
Sunderland were indebted to the brilliance of goalkeeper CraigGordon, who made fine saves from Jerome, Christian Benitez, StephenCarr and Liam Ridgewell.
Fraizer Campbell's 88th-minute strike ended the hopes of Alex McLeish's men.
The Black Cats had been threatening to throw away all their goodwork from before the break in front of an increasingly anxious crowd of37,962.
But ultimately, they did enough to secure another three-point haul and have now lost just one of their last eight league games.
Defeat for Birmingham was a reverse of the result when the two sidesmet at St Andrew's in October and dented their hopes of closing the gapon arch-rivals Aston Villa in the top half of the table.
The game was Sunderland's fourth in succession at the Stadium ofLight, and with the previous three having yielded five points, managerSteve Bruce was keen to increase that tally to eight ahead of toughaway trips to Aston Villa and Liverpool.
Back in the game: Cameron Jerome coolly converts past Craig Gordon
By the time the game was just 11 minutes old, his team were ontarget to do just that, thanks largely to Bent's continued potency.
The £10million man's 19 goals before kick-off represented a majorcontribution to the 31 points the Wearsiders had collected from theirfirst 29 games of the campaign.
However, a lightning quick double took him past the 20-goal mark,the accepted line which separates the top strikers from the rest, andput the home side firmly in control.
His first came after just five minutes when Benjani Mwaruwari,making his first start for the club in place of the injured KenwyneJones, saw his effort blocked and Bent rattled home the rebound fromclose range.
The second six minutes later was all his own work as he ran on toSteed Malbranque's fine pass and cut inside defender Scott Dann beforerifling a low drive past the helpless Hart.
It could have been 3-0 with 15 minutes gone but for Hart's savefrom Campbell, and it was then that Birmingham belatedly launched theirfightback.
Clinical: Darren Bent slots the opener
Twice inside a minute, Gordon had to dive full-length to turn awayKeith Fahey strikes, and the Scot just managed to get enough onBenitez's 30th-minute header to tip it against the crossbar.
But the home side finished the half once again in the ascendancy,with Malbranque blazing over after Hart had palmed away KieranRichardson's curling shot and then firing just wide from distance. In ascrappy start to the second half, City were making a fist of it onceagain, although without making much of an impression going forward.
There was warm applause and even a song for former Sunderland heroKevin Phillips as he warmed up on the sidelines, but it was Jerome whoshould have breathed new life into Birmingham's efforts to dragthemselves back into the game with 54 minutes gone.
He ran in behind defender John Mensah, who had earlier requiredtreatment for a knee injury, and went one-one-one with Gordon, but thegoalkeeper came out on top as he blocked with his legs.
Gordon excelled himself once again with a fine 59th-minute doublesave from Benitez and Carr, but he was finally beaten on the hour ashis defence deserted him.
Jerome got in down the right and powered his way into the box before chipping a shot over the helpless Scot.
Birmingham would have been level 11 minutes later had Gordon notmanaged to claw away Ridgewell's goal-bound looping header, and fellowdefender Roger Johnson headed wastefully over from the resultingcorner.
Arms race: Birmingham's Barry Ferguson (left) and Sunderland's Steed Malbranque battle it out
There was a collective sharp intake of breath when Phillips, a 59th-minute replacement for Benitez, looped a header onto the roof of the net with nine minutes remaining.
Bent might have calmed the nerves when he went through with 86 minutes gone and lifted the ball over the advancing Hart, but his effort lacked pace and Johnson was able to get back to clear.
However, Campbell sealed the win when he met substitute Jordan Henderson's cross to finally kill off the visitors.
Kevin Phillips tells Birmingham to splash the cash or risk relegationPhillips has 250 goals in his sight but won't celebrate at SunderlandSUNDERLAND AFC