Dennis Wyness scored what proved to be the winner after 73 minutes but Falkirk had former Scotland defender Jackie McNamara sent off earlier in the half.
In a low-key opening, it was St Mirren who threatened first when Stephen McGinn threaded the ball through to Andy Dorman but he shot a couple of yards wide.
Burton O'Brien tried to wriggle is way into the area but was halted by a solid tackle from Hugh Murray.
McNamara almost played in Michael Higdon but Jack Ross did enough to knock the striker off his stride and the chance was lost.
A lazy header from Dean Holden presented Dorman with chance midway through the first half but, although his shot beat Robert Olejnik, Lee Bullen was on hand to block.
Falkirk players were more often than not receiving the ball with their backs to the goal and there was a lack of urgency in the final third.
Kevin McBride livened things after 25 minutes when he took a short pass from O'Brien and rifled a 25-yard shot off the top of the crossbar.
But Olejnik came to his side's rescue with a magnificent double save shortly afterwards as Bullen failed to cut out a deep cross and McGinn had time to pick his spot.
The midfielder did not do too much wrong but the goalkeeper stuck out his right hand to keep it out and then blocked the follow-up effort with his legs.
St Mirren snatched a deserved lead four minutes from the break. Olejnik could only palm out a low show from Ross and Jim Hamilton was on hand to provide a simple finish from five yards.
Falkirk tried to hit back immediately and McBride saw a deflected shot thud against the top of the crossbar again.
McNamara was shown a straight red card for Falkirk ten minutes into the second half for a challenge on Hamilton out on the touchline.
But it was ten-man Falkirk show equalised after 62 minutes as Steve Lovell turned the ball in at the back post after Higdon had headed on a free-kick.
Wyness then put Saints back in front when he slid the ball home from close range after Dorman.
Lovell saw a shot hit the post and then Ross cleared a Higdon shot off the line in a goalmouth scramble in the closing minutes.