Billy Mehmet's first-half strike and Andy Dorman's 73rd-minute clincher took the battling Buddies off the bottom of the table, two points above the Bairns who could have secured their safety with a win.
Falkirk finished with 10 men after veteran defender Steven Pressley was shown a straight red for a clash with Mehmet but they have more on their plate now after a disappointing display.
John Hughes's side travel to Inverness next Saturday with both sides in the frame to go down, while St Mirren could also still find themselves in the Irn-Bru First Division if they lose to Hamilton and the result at the Tulloch Caledonian Stadium goes against them.
However, the Paisley side picked the right day to produce arguably their best display of the season.
St Mirren fans had turned up in good numbers to add to a cracking atmosphere and they watched their side dominate the first half from the whistle.
There were only seven minutes played when Falkirk keeper Dani Mallo made a great fingertip save from Jack Ross's 25-yard drive which was heading for the postage stamp.
From Garry Brady's corner, Saints defender Will Haining cracked the bar with a header, with the home side glad to eventually clear the danger.
Then in the 11th minute, with the pressure on the Falkirk goal incessant, Gary Mason's curling effort from the edge of the box took a deflection off Bairns skipper Darren Barr before Mallo forced it round the post for another corner.
The home survived the set-piece again but they continued to live dangerously. Moments later, Dorman's right-footed drive from 12 yards was heading for the net until it cannoned off Falkirk defender Tam Scobbie.
It was one-way traffic and in the 15th minute Ross took a Mehmet pass and smashed a shot from 20 yards off the upright and behind.
The Falkirk fans urged their team forward but St Mirren, dogged, determined and clearly fighting for their SPL lives, refused to let go their grip on the game.
After Bairns midfielder Scott Arfield had felled Brady 25 yards out, Dorman's free-kick beat Mallo but also sped past the wrong side of the post.
The onslaught continued. Just after the half-hour mark, as the Paisley side again broke with purpose, Dorman turned Pressley inside out before firing an angled shot high and wide.
In the 37th minute, when it looked like the home side had weathered the storm, St Mirren got the deserved breakthrough through Mehmet with Pressley at fault.
The Scotland coach's intended header to Scobbie was intercepted by Hugh Murray who helped it quickly onto the former Dunfermline player, who raced clear of Pressley before sliding past Mallo.
The goal failed to spur the home side into action home and St Mirren remained on top until referee Chris Boyle's half-time whistle offered the Bairns some respite.
Carl Finnigan replaced Neil McCann but it had no immediate effect as the game, albeit more sedate than the first half, was still being controlled by the visitors.
As driving rain began to fall, Falkirk carried on with their route-one football which gradually began to give them a foothold in the game.
However, the St Mirren defence held firm any time they were attacked, typified in the 61st minute when Burton O'Brien ended a half-hearted spell of pressure from the home side when he fired wide from the edge of the box.
St Mirren should have made it 2-0 in the 64th minute when Ross stood up a great cross to the back post, but although Mehmet rose to meet the ball unopposed, his header from six yards out was saved by Mallo.
Falkirk raced down the park and seconds later, with their first real chance of the game, Stevie Lovell's close-range drive was blocked by St Mirren keeper Mark Howard.
Saints were running the show and Mehmet's over-head kick from near the bye-line almost looped over Mallo, the Falkirk keeper having to stretch to clear.
But in the 73rd minute there was to be no let-off for the home side as Dorman carved open the Bairns defence to make it 2-0.
The Saints midfielder sent Mehmet racing down the left with the Falkirk defence claiming offside, and when the ball was cut back, Dorman slid the ball in from eight yards out.
Two minutes from time Pressley, ended a miserable day when he was shown a straight red card after clashing with Mehmet.
The final whistle saw the Saints players celebrate with their jubilant fans and although they are still in danger, on this performance who would bet against them surviving the drop?