It was far from a classic encounter with the sides locked at 0-0 until Andy Dorman scored the only goal of the game eight minutes from time to seal victory for the Buddies.
Falkirk had dominated for much of the match but woeful finishing was their downfall as they failed to convert their chances.
Michael Higdon tested Mark Howard early on with a right-foot volley that the ex-Falkirk man saved well.
Saints were producing good build-up play but struggling to break into the Falkirk area with Arnau Riera working well in front of the back four.
Ryan McCay got a long-range effort in that went over the crossbar before Billy Mehmet tried his luck from 25 yards out. His effort bobbled across the area before being gathered easily by Shane Supple.
Scott Arfield showed why he was nominated as young player of the year when he made a 40-yard run into the box before lofting his shot just over the crossbar much to the dismay of team-mate Higdon who had been yelling for the pass.
On the stroke of half-time Jim Hamilton produced St Mirren's best chance of the match blasting in a left-footed shot that just grazed the crossbar.
Falkirk made an enforced change during the break following an injury to Jack Ross and Pedro Mountinho made an immediate impact after being brought on to replace him. He linked up well with Arfield to give the visitors more dynamism up front.
Carl Finnigan wasted an excellent chance for the Bairns firing wide after creating the chance for a clear shot at goal while Riera also tried his luck but his weak shot did not threaten Howard.
Despite all of their pressure Falkirk's finishing let them down, Moutinho's directionless effort being symptomatic of his side's problems.
The match turned when Riera was sent off on 75 minutes for a high tackle on Dorman. With Falkirk down to ten men St Mirren lifted their game and it was Dorman himself who made the breakthrough with less than ten minutes remaining.
He scored his fifth goal for the Love Street outfit with a left-foot volley that flew high past the goalkeeper before nestling in the back of the net.