Once again he proved his value to Birmingham with another late goal to give the St. Andrew's side a vital win to take them a step closer to Premiership revival.
Horsfield took over late in the game from the injured Clinton Morrison and responded in fine style when he headed home following a shot from Stern John.
Early play, however, reflected the current positions of both clubs. There was very little to choose between either team in a very low key match which never reached the Premiership standard.
Unfortunately the early miss by Stephen Clemence in only the third minute set the general tone for the game.
Birmingham were on top in this early spell without being able to dominate the opposition. Although Albion were fighting for their Premiership survival they were lacking in several key departments. Neither Jason Roberts or Danny Dichio could raise their game as they were generally kept in close check by Kenny Cunningham and Matthew Upson.
The midfield industry of Jason Koumas eventually enabled Albion to move forward with more confidence and a quick cross field pass form Derek McInnes opened up an opportunity for Ifeanyi Udeze but his final shot went well wide.
With the game struggling to reach any great heights Birmingham's best chance in the first half occurred when a Robbie Savage pass was turned inside by Christophe Dugarry into the path of Clinton Morrison.
The Republic of Ireland international, who has struggled to make an impact since his move from Crystal Palace, stumbled and knocked his shot wide of the post. Neil Clement threatened for Albion with a 35 yard free kick but it was sadly off target which aptly summed up a disappointing local derby.
Although Dugarry displayed several neat touches, Birmingham missed the wing skill of the absent Stan Lazaridis.
The start of the second half was much brighter with Koumas darting through to force goalkeeper Ian Bennett to spill the shot. From the Koumas corner Dichio found Roberts, who headed the ball against the crossbar.
A couple of minutes later Dugarry should have scored his first goal for Birmingham. The Frenchman headed Jamie Clapham’s cross downwards but straight at Hoult.
Birmingham lost Morrison with a shoulder injury and he was replaced by Horsfield. Horsfield's strong running made a marked difference and only a goalline clearance by Neil Clement prevented Dugarry recording his elusive goal.
Birmingham's best chance was wasted by substitute Darren Carter when the youngster headed wide from a Savage cross.
But in a hectic finish Horsfield, at the far post, headed home his last minute winner following a shot from Stern John.