But while Bent's double, taking his return to 21 with five in his last three matches, paved the way, Birmingham would have clawed themselves level had it not been for goalkeeper Craig Gordon.
Gordon was simply outstanding from start to finish and when City scored on the hour through Cameron Jerome it was the Scotland No 1 who was unfortunate.
Gordon denied Birmingham time and time again, with Chucho Benitez, Jerome and Stephen Carr all thwarted before Jerome eventually gave the visitors hope.
The former Cardiff striker curled inside Gordon's far post after being sent clear down the right flank by a clever pass from substitute James McFadden.
But despite plenty of forays forward from Birmingham in the remaining half hour, Sunderland held on to their one goal cushion courtesy of Bent's early heroics before Fraizer Campbell added a third three minutes before time.
Bent's first arrived inside five minutes. He was on hand to strike low into Joe Hart's bottom left corner after Benjani's shot was deflected into his path.
And then Bent who could have hit his hat-trick late on - added his second six minutes later when he cut inside Scott Dann before finding the same corner again.
This was not as straight forward as it should have been for Sunderland, but with a tally of seven points from the last nine the probability of Premier League football staying on Wearside has increased again.
SUNDERLAND (4-4-2): Gordon; Hutton, Turner, Mensah (Bardsley 72), Ferdinand; Campbell, Meyler, Richardson, Malbranque (Cattermole 80); Benjani (Henderson 72), Bent.
Subs (not used) Carson, Zenden, Da Silva, Kilgallon.
BIRMINGHAM CITY (4-4-2): Hart; Carr, Johnson, Dann, Ridgewell; Fahey (McFadden 60), Gardner, Ferguson, Madera (Bowyer 72); Chucho (Phillips 60), Jerome. Subs (not used): Taylor, Larsson, Parnaby, Vignal.