It was a chance to secure their safety, but instead Raith Rovers produced a pitiful performance in Paisley worthy of relegation.
Raith fans may have feared the worst heading to an in-form St Mirren side needing a positive result to keep their Championship fate in their own hands.
However, the 5-0 defeat was beyond even their worst nightmares and leaves them requiring a victory at home to Ayr United next Saturday, and a defeat for St Mirren at Hibernian, or the Kirkcaldy side will face a relegation play-off.
And in that scenario, who would back Raith now? If this is what they produce when everything is on the line, then what chance in a play-off?
Worse still, it's not the first time this Rovers team has rolled over when the chips were down. We thought it couldn't get any worse than than the 4-0 horror show in Dumbarton. We were wrong.
And just to compound the utter embarrassment, Lewis Vaughan's goal for Dumbarton at Tannadice this evening earned them a 2-2 draw, which barring a ridiculous goal difference swing, all but guarantees their place in the division, at the expense of his parent club.
The post-mortem can wait for now - the season is not dead yet - but if they do go down, there will be lots to answer for.
With Iain Davidson ill, and Ross Matthews suspended - two whole-hearted players you would want for a match of this nature - Raith were left without a defensive bone in the midfield body.
Manager John Hughes had to assess his limited options and name a team capable of restricting a confident St Mirren, but also hurting them.
He did neither. The team selection and tactics simply did not work but that does not excuse the players' performance.
It was a coward's display lacking in courage and conviction, the complete opposite of the second half efforts at Easter Road in midweek.
St Mirren played with heart and skill, and were on top from the first whistle to last, but the first goal would always be crucial.
It arrived on 27 minutes when Raith failed to recognise the danger as a high punt wasn't dealt with, allowing Cameron Smith to hit the by-line and cross for Stephen Mallan, completely untracked, to fire home from close range.
Raith had started like a team without a cohesive game plan, but after losing the opening goal, they descended into a ramshackle outfit.
It was 2-0 on 39 minutes when Rudi Skacel, who was chasing shadows, conceded a free-kick 25 yards out.
Mallan's effort did not look too troubling until the wall splintered, and a huge deflection wrong-footed Pavol Penksa and spun into the net.
Raith were lucky to get to half-time without further damage as a couple of Saints' efforts from outside the area whizzed narrowly off target.
Inexplicably, the same 11 players returned for the second half, and Raith fell three behind just two minutes after the restart when Gary McKenzie nodded down a corner and Rory Loy was quickest to react to flick the ball home from six yards.
It was game over - for Raith at least - because St Mirren were far from finished.
In the immediate aftermath of conceding a third Hughes made a triple change - bringing on Jonny Court, Ryan Hardie and Mark Stewart - but it was too late now. At least two of the three should have started.
The subs give Raith an immediate spark as Kevin McHattie registered the first, and only threatening shot of game from the visitors, as his shot from 12 yards rippled the side-netting.
But it didn't last and St Mirren were soon on the hunt for a fourth with Penksa tipped over a Mallan volley at full stretch, but there was nothing the 'keeper could do as the midfielder completed his hat-trick in spectacular style on the hour mark.
There was no Raith player within several yards of Mallan as he looked up, and steered a simply stunning effort into the top corner from 30 yards.
That was the cue for almost half of the 450 travelling Raith fans to begin their journey home, and those that remained summed up the dire situation with the chant, "You're nothing special, we lose every week."
It was now just a case of reaching full-time with no further damage to an already unhealthy goal difference, but even that was beyond the away side as St Mirren added a fifth in the closing minutes, Lewis Morgan drilling past Penksa after Mallan again found space to set up the chance.
Raith will need a big performance next week - and some luck - to get out of trouble. They could do with taking a lesson from Craig Easton and his boys.
St Mirren: O'Brien, Eckersley, Mackenzie (Webster 85), Magennis (Quinn 80), Morgan, Irvine, Baird, Mallan, Loy, Smith (Demetriou 76), McGinn. Not used: Langfield, Sutton, Fjelde, Todd.
Raith Rovers: Penksa, Thompson, Thomson, Skacel (Stewart 52), M'voto, Johnston (Court 52), C.Barr, B.Barr (Hardie 52), McManus, Handling, McHattie. Not used: Brennan, Court, Benedictus.
Booked: Skacel, Handling, McManus.
Referee: Kevin Clancy