The 15-point lead Danny Lennon's men once enjoyed over Clydesdale Bank Premier League basement boys Dundee has been slashed to just eight, and could be cut to five with two games to play if John Brown's side beat Aberdeen on Sunday.
It was a woeful display from the Paisley outfit as they conceded after Jamie Walker's opener just 14 minutes in.
Kevin McHattie's free-kick 60 seconds before half-time gave them an uphill task in the second period but a spirited opening five minutes was quickly quelled when Kenny McLean was sent off after barging into the scorer of the second in the box, leaving Jamie Hamill to wrap up the win from the penalty spot.
Brown has questioned Saints' bottle, and if his side beat the Dons at home, the Buddies will have to prove their mantle by beating either Aberdeen at home or Kilmarnock away if they want to retain possession of their SPL destiny.
The signs were ominous for Lennon's side as early as the third minute.
A frantic start saw John Sutton go close for the hosts while Saints keeper Craig Samson did well to adjust and block Jason Holt's drive five minutes later after it took a nick off Marc McAusland.
But the Paisley number one could do nothing with Walker's strike just before the quarter hour.
The 19-year-old robbed Esmael Goncalves just inside St Mirren territory and drove 25 yards up field unchallenged before smashing the ball off the underside of the bar and into Samson's net from the edge of the box.
On-loan Newcastle left-back Paul Dummett then survived a penalty appeal for St Mirren after appearing to handle in the box while Dale Carrick sent a half-volley over the bar as the Buddies failed to clear Hamill's launched free-kick after 25 minutes.
The visitors seemed unable to repel the continuous waves of maroon attacks and were hardly helped when right-back David Van Zanten was caught napping as the ball rolled towards the touchline.
Walker was alert again and pinched the ball but could only find the side-netting after darting into the area.
With no sign of improvement and Van Zanten on a booking after fouling Walker, Lennon tried to take the initiative by replacing the Irishman with striker Sam Parkin and moving midfielder Conor Newton back into defence.
But worse was to come for the Paisley men as McHattie rifled his 35-yard free-kick home a minute before the break.
It was soft award after McAusland tussled with Carrick but the strike went through the defensive wall and Samson was slow to get down.
Saints had managed just an off-target shot from Lee Mair in the first half but finally tested Jamie MacDonald five minutes after the interval as Newton slammed a shot against the keeper from 12 yards on the angle.
Holt was then required to head off the line as Parkin met Gary Teale's corner with a thumping header.
But just as Saints looked set to finally give their opponents a game, the match was over as a contest. The away defence was at sixes and sevens as Ryan Stevenson played in McHattie and McLean was forced to barge the young defender over just as he was about to pull the trigger.
Referee Brian Colvin pointed to the spot and produced a red card for the perpetrator, leaving Hamill to ram home his first goal of the season with the successful penalty.
A chink of light appeared on the hour mark when MacDonald was caught out of position as he saved Paul McGowan's low shot, but it was snuffed out when Hamill got back on the line to head Teale's lobbed effort from the rebound clear.
The right-back was there again to deny Saints as he headed away just as Parkin looked to tap into another empty net when MacDonald was clattered by Steven Thompson as they challenged for a high ball.
The demotion drama now moves to Tayside but Aberdeen can do Saints a favour by killing off Dundee's revival and sending them back to the Irn-Bru First Division by avoiding defeat.
- Sports & Recreation
- Jamie Hamill
- Danny Lennon
- Craig Samson