Andy Dorman (left) scored his fourth SPL goal in as many games
St Mirren killed off Motherwell's hopes of a top six place thanks to late goals from Steven Thomson and Andy Dorman.
Motherwell enjoyed the bulk of play and Mark Howard made good saves to deny David Clarkson and John Sutton, while Stephen Hughes hit the post.
Craig Dargo burst free late in the first half but the St Mirren striker's shot was blocked by Graeme Smith.
Thomson tapped in after a mistake from Mark Reynolds and Dorman slammed home a screamer from 25 yards to settle it.
Trailing sixth-placed Hibs by a point, Motherwell needed a victory to put pressure on the Edinburgh side, who host Rangers on Sunday.
However, Mark McGhee's men never found any passing rhythm and were punished by two late goals from the Paisley side, who now move seven points clear of bottom club Falkirk.
Following a cautious opening, Motherwell enjoyed a spell of sustained pressure that began with David Clarkson's stinging shot from the edge of the penalty area, which was beaten away by Buddies keeper Howard.
John Sutton then sent a looping header wide from a good Paul Quinn delivery before Hughes and Jim O'Brien saw efforts from long range narrowly miss the target.
St Mirren took 25 minutes to pose any threat and Stephen McGinn's tame strike from distance caused no problems for Smith in the Motherwell goal.
O'Brien later tried his luck with a chipped effort on the angle but it was well held by Howard.
In the final minute of a ragged first half, Dargo broke clear of the home defence but Smith raced from his line and spread himself well to block the striker's low shot.
After the interval, Howard had to look smart to keep out another fierce strike from Clarkson and Hughes fired wide after creating an opening with some fancy footwork.
But industrious St Mirren continued to pack men behind the ball and the home side could little room in which to manoeuvre.
O'Brien did find a way past an opponent on 67 minutes and sent over a great cross for Sutton to power a header on target, only for Howard to make an acrobatic save.
'Well defender Brian McLean then headed over from close range as he glanced on a dangerous in-swinging free kick from O'Brien.
With 13 minutes remaining, St Mirren stunned the hosts with a breakaway goal that owed much to some disastrous defending.
Dargo met a long, high ball forward and lofted a first-time pass into the danger zone, which Reynolds could only scuff into the path of Thomson and the Buddies midfielder could not miss from a few yards out.
With Motherwell steaming forward in search of a goal, Dorman was afforded time and space to send a sweet strike whizzing past Smith from long range to make it four goals in four games for the Buddies midfielder.
During a desperate penalty box scramble, Hughes sent a glancing header against the post but Motherwell could find no way back.
Motherwell manager Mark McGhee on missing the top six: "I'm disappointed it has happened the way it happened.
"Even if Hibs had won tomorrow, I think we could have borne that given the time we had in November and December with injuries, suspension and illness.
"I keep saying it and I'm fed up saying it but I've got to remind people of it because people I've already spoken to are saying we had a poor start to the season.
"The poor start was about our small squad, but we have bounced back since Christmas and only lost three in 15, which isn't bad and put us in a good position.
"And to do as well as that and then just to throw it away the way I feel we did today... because I think that's what we did.
"I'm not taking anything away from St Mirren. But we dominated the first half and we still didn't produce enough quality to win the football match."
St Mirren manager Gus MacPherson: "It's a vital three points but nothing has been decided.
"This was probably not the prettiest performance in recent months but we got the job done.
"On surfaces like that, game plans go out the window.
"You have to play in a cup-tie manner. It's a case of defending properly and trying not to make too many mistakes, and trying to get the opposition defending as much as they can."
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