St Mirren's winning run came to a shuddering halt on the road as they suffered a heavy defeat in Perth.
Stephen Robinson's side were hoping to make it four Premiership wins on the bounce after an excellent string of performances.
But their hopes of achieving that suffered an early blow when debutant Nicky Clark made an instant impact for his new side, netting a first-half opener from close range after some woeful defending.
Drey Wright put the hosts further ahead, with the Buddies millimetres away from cutting the deficit when Scott Tanser hit the bar and Jonah Ayunga's follow-up smacked the post.
The hosts put some gloss on the scoreline through Graham Carey's late penalty, with the Paisley Saints returning home empty-handed and now looking to start a new run against Hearts on Sunday.
St Mirren were unchanged from their impressive win against Hibs, with Ayunga and Curtis Main still the preferred front two with Eamonn Brophy and Alex Greive having to make do with spots on the bench.
St Johnstone threw their new signing Clark straight into the action, pairing the former Dundee United striker with Stevie May as the hosts hunted more goals.
St Mirren started the game fairly brightly, with Ayunga almost getting on the end of a teasing Ryan Strain cross as Stephen Robinson's men seeked an early opener.
Ayunga did get on the end of a Main cross shortly after, but he swivelled and fired well wide of Remi Matthews' goal.
There was an early scare for the healthy away support when Strain looked to have twisted his knee. After receiving some treatment the Aussie was given the all clear.
On the 25 minute mark the hosts took advantage of some slack St Mirren defending to break the deadlock. Drey Wright's cross picked out May at the back post. He nodded it back across goal to the unmarked Clark, who made no mistake as he fired in from close-range.
Charles Dunne went into the book for persistent fouling as the visitors struggled to gain a foothold in the game.
Keanu Baccus was the Buddies brightest spark in the first half. The swashbuckling midfielder picked out Ayunga who fed the ball to Main in a perfect position on the edge of the box.
The powerful striker's first touch let him down, with the St Johnstone defence recovering to snuff out the danger.
Defender Fraser proved an unlikely attacking threat as he burst through the midfield. The centre back couldn't quite pick out Main as yet another St Mirren attack broke down at the vital moment.
The visitors returned unchanged after the break and could have levelled almost immediately.
Mark O'Hara picked a defender's pocket on the edge of the box and burst in on goal. His cross to the near post was snuffed out, with the midfielder missing his unmarked teammates in the box.
With half an hour left to rescue something from the game Robinson turned to his bench, with Brophy, Tanser and Jay Henderson replacing Richard Tait, Main and Strain.
The subs almost had the opposite effect when St Johnstone skipper Melker Hallberg curled a shot just over the bar with Trevor Carson scrambling.
Ayunga blasted over from the edge of the box, with Henderson drawing a comfortable save from Matthews as St Mirren started to operate more of a shoot on sight policy.
Unfortunately for the Paisley Saints it was the home side that rippled the net next.
Adam Montgomery burst past Henderson before cutting back to Jamie Murphy. He in turn fed Wright who calmly slotted his side further ahead.
Striker Greive entered the fray as Robinson went for broke in a bid to get something from the trip north.
Declan Gallagher wasn't the man St Mirren wanted the ball to fall for when Tanser's cross eventually reached the back post. The centre back fired well wide with the goal at his mercy.
The Paisley side couldn't have come any closer to getting one back with quarter of an hour left on the clock when Tanser's header crashed off the underside of the bar. Ayunga then proceeded to hit the post with the rebound .
It went from bad to worse for the Buddies when Fraser gave away a late penalty for a reckless tackle on sub Connor McLennan.
Former St Mirren midfielder Carey rubbed salt in the wounds by blasting into the top corner to seal all three points.
