How often are teams like St Mirren supposed to be the subject of ritual slaughter at the hands of Rangers or Celtic? Too often, is the answer, for the good of the Scottish game, which is why this gritty but unattractive game at Love Street yesterday suddenly became something worth cherishing for Gus MacPherson, his team and those legions of long-suffering Buddies.

They will long remember what occurred after 76 minutes yesterday, and Stephen McGinn, the 20-year-old scorer of St Mirren’s winner, certainly will. McGinn, the grandson of the former Celtic chairman, Jack McGinn, had only been on the park for four minutes, having replaced Andy Dorman, when his left-foot curler from 25 yards swerved sweetly and mesmerically inside Allan McGregor’s right post, leaving the recalled Rangers goalkeeper utterly helpless.

It was, in truth, one of those goals that may prove a once-in-a-career moment for a player like McGinn. The flight of the ball couldn’t have been better struck from a not-quite-central position, thus completely baffling McGregor and leaving him motionless. It was more a rare work of art from McGinn than a failing of the Rangers goalkeeper.

What followed that 76th minute, though, was folly and near-suicide from St Mirren. The home side, having claimed their lead, showed no poise or calm whatsoever, repeatedly booting the ball up the field, even when fully 12 minutes were remaining. And the upshot of their near-ruinous lack of composure was an utter siege around Mark Howard’s goal and chance upon chance for Rangers.

Kris Boyd, on as a substitute, hit a post with a header. Then Davie Weir, with a decent chance, slammed the ball over the bar. Then a Kirk Broadfoot header was cleared off the line by Will Haining, and Boyd and Kenny Miller both had shots scrambled clear by St Mirren’s frantic defence. There were a few moments when it was like the last days of Stalingrad as St Mirren came close to disintegrating.

When Broadfoot then charged down the right and appeared to be tripped in the box by Gary Mason, the travelling Rangers support shrieked for a penalty, as did the visiting bench. But Willie Collum, the referee, waved aside the claims, and St Mirren staggered home. It was their first victory over Rangers at Love Street since April 1986 and, with Saints soon to decamp to a new ground at Ferguslie Park, it will certainly be their last.

“We showed effort and commitment, and we defended properly, albeit with a bit of luck towards the end, to get the points,” MacPherson said. “But it was a great finish from Stephen McGinn – from where we were on the touchline, we got a perfect view of it, and it was going in all the way.”

Walter Smith lamented Rangers’ impotent display, or at least the first 70 minutes of it, when his team fairly blustered around. “We didn’t really create that many scoring opportunities and it was only in the last 20 minutes that our pressure became more forceful,” Smith said. “We just hadn’t been incisive enough.”

To add to Rangers’ woes, Kevin Thomson limped off with a swollen ankle and is now a doubt for Scotland’s World Cup qualifier against Norway at Hampden on Saturday.

Ironically, the game was graced with the sudden reappearance in Rangers’ goal of McGregor, a player whom Smith had dropped for – how shall we put this euphemistically? – living the high life. Around Glasgow tales of the likeable McGregor’s prodigious socialising are legion and it prompted from Smith a faintly disapproving puritanism in dropping the player two weeks ago.

Yesterday, however, such was the magnificence of McGinn’s goal, the goalkeeper looked fairly blameless, and will surely hold his reclaimed place. “It’s a bit hard on Neil Alexander but maybe I was a bit unfair in throwing Allan in so early this season after he had been out injured last season,” Smith said. “But he’s now had three weeks of intense training and I think he’s got his sharpness back.”

It was a rich irony that, on the very day that McGregor was recalled, a Sunday tabloid led with a front-page yarn about further exploits by the goalkeeper, this time involving a loud girlfriend and even louder music, which apparently has been annoying his neighbours. Poor Smith must have his head in his hands, not in prayer, but at the mere mention of this character.

There was much about St Mirren’s play to be commended. MacPherson’s men might have been rooted to the bottom of the league but, in the first half especially, they opened the play up well and tried to take the game to Rangers, without actually creating real chances. Players like Mason, Garry Brady and Billy Mehmet have journeymen status in the SPL, and out of economic necessity are MacPherson’s working tools, but they still played with imagination as they tried to stretch Rangers over the park.

Rangers struggled for most of the afternoon to slice through their opponents, but still could have taken the lead after 62 minutes when a lovely move involving Steven Davis and Pedro Mendes released Jean-Claude Darcheville in the inside-right channel. But as he had done once or twice in the match, the Frenchman somehow managed to scuff his shot harmlessly wide of Howard’s left post.

In Darcheville’s play, Rangers’ entire afternoon was summed up. Shortly after his miss, the striker was taken off, replaced by Boyd, before McGinn’s stunning strike. Then came those eye-popping closing minutes in which St Mirren survived, only just.

St Mirren (3-5-2): M Howard 5 S Cuthbert 6 J Potter 7 W Haining 7 S Robb 6 J Ross 6 A Dorman 5 G Mason 5 G Brady 6 B Mehmet 5 D Wyness 4 Substitutes C Dargo 4 (for Wyness, 72min),S McGinn 7 (for Dorman, 72). Not used C Smith, H Murray, T Brighton, J Hamilton, M McAusland.

Rangers (4-1-3-2): A McGregor 5 K Broadfoot 6 D Weir 6 M Bougherra 6 S Papac 5 K Thomson 5 S Davis 6 P Mendes 6 C Adam 4 J-C Darcheville 5 K Miller 5 Substitutes K Lafferty 4 (for Thomson, 58min), K Boyd 6 (for Darcheville, 63). Not used N Alexander, D Beasley, C Dailly, A Niguez, R Loy.