Clayton Donaldson came back to haunt his old club when he earned the penalty that Paul Caddis calmly stroked home for the visitors as Bees skipper Tony Craig saw red for the foul.
But Brentford dug deep after the break and fully deserved to level when substitute Moses Odubajo raced the length of the City half and chipped over goalkeeper Darren Randolph.
It was no more than the home side merited after weathering a torrid first half in which Birmingham created chance after chance.
Wes Thomas fired wide from the edge of the box with 19 minutes gone and somehow contrived to head David Cotterill's pinpoint cross over from almost under the bar.
Bees keeper David Button kept his side in it on the half hour, first tipping Cotterill's curling drive for a corner and then producing a reflex stop to deny Lee Novak's close-range header.
Brentford's Andre Gray wasted a golden chance to equalise on 50 minutes but failed to get the decisive touch on Alan Judge's deflected cross.
Brentford had strong shouts for a penalty waved away by the referee when Jake Bidwell cut in from the left only to be clipped by Blues centre-back David Edgar.
Judge tested Randolph at the foot of his post but it was the introduction of Odubajo that changed the game.
He raced clear of a flat-footed Blues defence to clip the ball home and send the Griffin Park crowd into raptures and, after that, it was always the Londoners who looked more likely to take the points.
Substitute Stuart Dallas almost won it for the home side late on but his fierce drive cutting in from the right was saved at the second attempt by Randolph.
Blues will be disappointed they didn't match the intensity of the first half and dropped deeper and deeper as the game wore on.
Lee Clark took Donaldson off midway through the second half and the visitors' pressing game dropped off in a move that his opposite number Mark Warburton exploited with the pace and trickery of Odubajo.