Livingston's top six hopes took a severe dent thanks to a woeful first-half display in Paisley as they fell to a 3-0 loss.
The Lions conceded a penalty barely 20 seconds into the clash with Mark O'Hara slotting home following a ridiculously long VAR check. The visitors never recovered as they found themselves three down at the break.
Luiyi de Lucas - at fault for the first goal - lost his marker to allow Tony Watt to tap home the Buddies' second on 18 minutes before O'Hara casually rolled his second spot-kick into the net just before the break to give Stephen Robinson's men a deserved healthy advantage.
Livi did at least offer something resembling a fightback after the interval with a number of chances squandered but the damage was well and truly done by that point and they were unable to reduce the deficit.
Livingston boss David Martindale named two changes to the side that defeated Ross County last time out with Kurtis Guthrie handed his first league start since October. Jack Fitzwater returned from suspension to slot into a back three as the Lions changed formation to a 3-5-2. It's fair to say it didn't work.
The Lions got off to the worst possible start when de Lucas appeared to handle inside the box in the opening seconds. A lengthy VAR check finally awarded the Buddies the spot-kick with O'Hara dispatching the ball beyond Shamal George.
That rattled the visitors who struggled to get a foothold in the early proceedings with the Buddies enjoying plenty of possession and peppering the Livi box with deliveries. It paid dividends just before the 20 minute mark as the hosts doubled their lead.
De Lucas's poor start continued as Watt escaped his man inside the box to get on the end of Curtis Main's pass before calmly rounding the keeper and rolling the ball into the net.
Livi never really recovered but they did pass up a great opportunity to drag themselves back into the contest 10 minutes before the break as Nicky Devlin's enticing cross was headed over the bar by Joel Nouble.
Moments later it was well and truly game over after St Mirren were awarded a second penalty for another handball with O'Hara once again finding the net with ease for his 10th goal of the season.
It took until they were three goals down but the Lions did at least offer something in response as three chances either side of the break went begging. Andrew Shinnie saw his hooked strike bounce off both post and bar in the final stages of the first-half before Cristian Montano had his header clawed away by Trevor Carson before Devlin fizzed an angled drive inches wide of the far post in the opening exchanges following the restart.
Nouble squandered a glorious chance of the hour mark as a long ball forward saw him get beyond the last defender but he screwed his effort wide.
The remainder of the contest resembled little more than a pre-season friendly with a slew of substitutes disrupting any flow and both teams knowing the outcome had been decided.
For Livi - who have been so inexplicably inconsistent this term - a bounce back is desperately needed as they travel to Fir Park next weekend looking to kick-start their top six bid once again.
St Mirren: Carson; Fraser, Shaughnessy, Dunne; Strain, Baccus (Flynn 67), Gogic, O'Hara (Kiltie 45), Small (Tanser 83); Watt (Greive 67), Main (Offord 90). Subs not used: Urminsky, Taylor, Jamieson, Gallagher.
Goal: O'Hara (5 pen, 41 pen), Watt (18)
Livingston: George; De Lucas, Fitzwater, Boyes (Brandon 79); Devlin, Pittman (St. Kelly 61) Holt (Omeonga 45), Shinnie (Anderson 71), Penrice (Montano 45); Nouble, Guthrie. Subs not used: Hamilton, Bahamboula, Bradley.
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