Falkirk were put to the sword as the Saints camped in their half and it was a miracle that only Billy Mehmet's goal separated the sides at half-time.
But Andy Dorman stroked St Mirren to victory and left Falkirk teetering on the brink of a drop to the first division.
Falkirk's recent run had made them favourites, but in truth they froze and lost key battles all over the pitch which condemned them to defeat.
St Mirren were quickly out of the traps and were camped in the Falkirk half for the opening exchanges.
It took a wonderful save from Dani Mallo to keep the Bairns level as former Falkirk skipper Jack Ross latched on to Steven Thomson's lay off to hammer in a 20-yard half-volley.
It looked bound for the top corner until Mallo threw himself to his right to tip the ball over the bar.
From the resultant corner, Will Haining found space to power in a header from Garry Brady's cross that rattled the bar.
And nervy Falkirk were at sixes and sevens as Gary Mason picked up the ball 20 yards out and curled a right-foot effort towards the bottom corner, but again Mallo was equal to the task with a tip around the post.
It was one-way traffic and only a brave block from Tam Scobbie prevented Dorman's 15-yarder from finding the net before Haining wasted another opportunity by volleying Dorman's corner wide of the mark.
But it was the woodwork that came to the Bairns' rescue again after 15 minutes.
The right-back was finding acres of space to charge forward and his 20 yarder had bounced to safety off the post before Mallo had even moved.
And after 26 minutes, Haining was inches away with a glancing header from Dorman's free-kick and still Falkirk hadn't mustered a shot on target.
Finally, after 36 minutes, Saints got the goal their pressure so richly deserved.
Hugh Murray's through ball left Mehmet one-on-one with Steven Pressley and the defender's lack of pace was cruelly exposed as Mehmet left him for dead and slotted a cool finish past Mallo.
A torrential rainstorm seemed to blunt St Mirren's ambitions at the start of the second half as Falkirk slowly got more possession.
Michael Higdon even managed a couple of efforts at goal, albeit both his volley and header were well off target.
But worryingly for the home defence, they still looked all at sea every time St Mirren attacked.
This was never clearer than when Ross skipped to the byline to square for Mehmet, who was in acres of space six yards out.
He should have put the game beyond Falkirk but his header was straight at Mallo.
The keeper immediately launched the ball up the park and Steve Lovell found himself straight through on Mark Howard, but composure deserted him as he scuffed his shot into the keeper's outstretched legs.
Mehmet almost atoned for his earlier miss with a spectacular overhead effort that Mallo clawed out from under his crossbar.
But seconds later it was all over for Falkirk as Mehmet broke down the left and picked out Dorman's run with a measured cutback that the midfielder stroked home from 12 yards.
That led to a mass exodus of Falkirk fans and, to complete the misery, Pressley was given a straight red card for an elbow on Mehmet two minutes from time.