The home team appeared to be coasting to a comfortable win following David Cotterill’s first-half strike only for Assombalonga to score a fine equaliser in the 84th minute.
But five minutes later he was sent off after handling the ball in the area and Paul Caddis sent Forest goalkeeper Karl Darlow the wrong way from the penalty spot to secure three points.
Birmingham, celebrating the induction of former striker Trevor Francis into their Hall of Fame, responded to the occasion with a strong start against a Forest side buoyed by successive wins over Norwich and Wolves.
They played with a pleasing eager approach which had Forest on the defensive, teenager Demarai Gray proving particularly troublesome with his jinking style of play.
Gray confidently pulled the ball back on the edge of the penalty area, wrong-footing the Forest defenders in the process only to glide his side-footed shot wide of the post.
But Birmingham were ahead on 10 minutes, Cotterill’s superlative left-footed swinging shot arrowing into the top far corner of the net.
City’s high-tempo game kept Forest at full stretch and they had a lucky let-off in the 31st minute when a first-time cross by Caddis was deflected by Danny Fox onto the top of the crossbar with Darlow nowhere near the ball.
Former Birmingham favourite Chris Burke managed Forest’s only shot on target late in the first half but the visitors grew into the match in the second preiod, with Henri Lansbury playing a pivotal midfield role.
Birmingham weathered the storm and Clayton Donaldson went close to scoring only to be thwarted by a fine save by Darlow who was later very nearly caught out by a long-range lob by Cotterill from the halfway line.
Forest manager Stuart Pearce made substitutions to inject more life into his team and was rewarded when Assombalonga headed home after Jamie Paterson had flicked on Michail Antonio’s cross.
But having seemingly rescued a point Assombalonga was judged to have used a hand to keep out Michael Morrison’s goalbound header and had to watch from the sidelines as Caddis coolly converted the resulting spot-kick.