A goal in the 75th minute from substitute Garry O'Connor sealed the win for Alex McLeish's men, leaving the Tigers languishing in the relegation zone.
Stephen Hunt tried his luck early on for the home side as he rifled in a left-footed shot from 30 yards, but keeper Joe Hart was equal to the effort as he comfortably parried the ball to safety.
Christian Benitez then came close for Birmingham as his turn and shot inside the box had to be well saved by keeper Boaz Myhill who did well to turn the ball away down to his left.
Myhill was called into action again moments later as Blues midfielder Lee Bowyer found the space to curl in a right-footed shot which looked to be heading for the top corner, until the Wales international leapt to his left to cleverly turn the ball over the bar.
Benitez came within inches of opening the scoring on the stroke of half-time as he fired in a rasping left-footed effort from 20 yards, but again Myhill was on hand to neatly turn the ball over the bar.
Birmingham came close on the hour mark through Keith Fahey as Gary McSheffrey found the Irish midfielder with a pin-point cross from the left, but his header from eight yards flew just wide of the post.
Myhill had to be on his toes midway through the half, as a Blues corner landed at the feet of Bowyer inside the six-yard box and his low shot had to be superbly parried away by the Hull keeper.
Benitez was next to test Myhill as he managed to turn his man and race in on goal only to see his right-foot shot saved well saved by the Hull number one.
However, from the following corner the visitors did beat Myhill to break the deadlock, as an out-swinger from Fahey found the head of substitute O'Connor who made no mistake from six yards to make it 1-0.
Hull nearly found themselves level in the last few minutes as a corner from Hunt found its way to the head of Ibrahima Sonko, but the defender's close-range effort came back off the crossbar.