Goals from Ashley Barnes and David Lopez put the away side ahead after Steven Caldwell had headed the hosts in front.
It was the visitors who made the stronger start to the match when Lopez drove down the right with 11 minutes gone before delivering a low cross into the penalty area, but it was just too far in front of Andrea Orlandi.
The home side responded just a minute later when Chris Burke cut in from the right before curling a shot just wide of the far post.
It was Birmingham who took the lead after 18 minutes when captain Caldwell rose highest in the penalty area before heading the ball into the back of the net from Robbie Hall's corner.
Brighton made a positive response and came close to equalising just two minutes later, but Liam Bridcutt's curling effort from the edge of the box was just wide of the mark following David Lopez's corner.
However, the Seagulls did find a leveller in the 24th minute when Barnes played a one-two with Orlandi before directing his shot into the bottom corner past Jack Butland.
Birmingham nearly went back in front a few minutes later when Wade Elliott teed up Callum Reilly, but the latter shot over from 18 yards.
The away side came close to taking the lead in the 32nd minute, but Gordon Greer couldn't find the back of the net from Lopez's free kick.
However, Brighton did take the lead just moments before half time, when Wayne Bridge's cross was thumped home on the volley by Lopez.