In the 75th minute of a dour encounter, Bairns midfielder Arnau Riera was shown a straight red card for appearing to elbow Dorman and the home side soon took advantage of their extra man.
When David Barron's cross from the right was flicked on by Jim Hamilton, the former New England Revolution player hammered his shot past Falkirk keeper Shayne Supple for a victory that had not looked likely until the latter stages of the match.
Bairns boss John Hughes will consider his side unfortunate not to have taken anything from this game which never came to life until his Spanish midfielder was dismissed.
A first half of mind-numbing mediocrity began and ended with two incidents of note.
In the fifth minute Bairns' striker Michael Higdon caught a Carl Finnigan cross on the volley at the far post and Saints keeper Mark Howard was glad that the powerful effort arrived straight at him from around 10 yards out.
In the final minute, Saints striker Hamilton drove a shot in from 25 yards which clipped the bar on its way over.
In between, the fans who turned up for what was essentially a meaningless end-of-season game, saw little in the way of quality football.
Moments after his early effort, Higdon stretched to send in a header which Howard gathered easily while at the other end, Billy Mehmet had the first effort on target for the home side after 20 minutes with his low drive from 25 yards which was saved by Bairns keeper Shayne Supple.
There was a moment of humour when rookie referee Scott MacDonald awarded a free-kick to Falkirk then changed his mind, giving it to St Mirren to laughter from the home fans who had complained about the original decision.
But there was nothing entertaining about the football.
Falkirk had the slight edge in terms of their goal threat with Higdon's aerial prowess a continual threat. The big striker might have done better with Kevin McBride's 35th minute corner than bullet his header wide of the target from eight yards out.
Five minute from the break Falkirk midfielder Scott Arfield ended an impressive run towards the Saints box in disappointing fashion, blasting high over the bar.
Falkirk substitute Pedro Moutinho replaced the injured right-back Jack Ross at the interval, forcing Bairns boss Hughes in to a reshuffle with Arfield dropping back to defence. Saints started the second half with some vigour but still retained a disjointed look about them and they nearly paid the price five minutes after the restart.
Arfield's innocuous-looking through-ball went under the foot of Paisley defender Will Haining allowing Finnigan to drive in at goal but from 14 yards out he scooped his shot over the bar.
Finnigan did better when he robbed the careless Gary Mason on the right-hand side of the St Mirren box but his low drive across the goalmouth found no takers.
Falkirk were the more creative side but just after the hour mark, when Moutinho blazed his shot from 25 yards, it looked the game was destined to remain goalless.
The game burst in to life in the final stages and it was the Saints who ultimately came out smiling.
In the 73rd minute Craig Dargo missed the chance of the game when he turned Hamilton's cross from the right past the back post.
Two minutes later, Falkirk were reduced to 10 men when Riera was shown a straight red card for appearing to elbow Andy Dorman when they clashed on the halfway line.
More misery was to follow for the visitors when Dorman watched Hamilton's head-flick come out of the air before he slammed a volley past Supple from 16 yards out.
In the final minutes Dargo was lucky to escape with only a yellow card when he felled Falkirk defender Gerard Aafjes who had to be replaced by Chris Mitchell.