The much travelled striker was on the mark in the 62nd minute to relieve the growing pressure on manager Alex McLeish.
It was only Birmingham's eighth goal in their last 12 games and only their third success in this spell.
Importantly Birmingham are only three points behind the leaders Wolves following their defeat at Burnley.
Making his first start in six games, Bent rescued the game from the doldrums with his third goal of the season.
Birmingham's success was sealed by Keith Fahey in the 75th minute when the Irishman notched his first goal since his recent arrival at St Andew's.
Earlier Birmingham, as usual, were slow out of the blocks and it was significant that Nathan Tyson had more pace than Martin Taylor.
The Forest striker made an early break but ballooned the ball high over the bar, while on the second occasion his low cross from the left evaded everyone.
Forest played with a great deal of purpose against a Birmingham side that struggled to find their early season form.
Promotion chasing Birmingham suffered a big set-back when they lost the services of their leading marksman, Kevin Phillips, with a leg injury in the 31st minute.
Previously Phillips had recorded Birmingham's best chance to go ahead with a snapshot which was deflected just wide.
The best save of the first half came in the 36th minute when Paul Smith palmed out a Liam Ridgewell header from a Seb Larsson free-kick.
In the next minute a Marcus Bent header was deflected onto the bar following a Scott Sinclair corner.
Bent finally broke the stalemate in the 62nd minute when he forced the ball home from a David Murphy cross despite Wes Morgan making a desperate attempt to clear off the line.
The relief in the Birmingham side was evident and they celebrated with a sublime goal from newcomer Fahey in the 75th minute.
Collecting a pass from Cameron Jerome, he cheekily chipped a right footed shot over the head of Smith and into the net off the post.
Only a brilliant save by Maik Taylor prevented Tyson pulling back a goal.
Taylor had to fling himself to his left to keep out a powerful shot from the Forest striker.