Well went into this game knowing it was win or bust if they hoped to make the top six as anything else would hand the final place to Hibs.
In the end, second-half strikes from Steve Thomson and Andy Dorman ended their hopes, but at the same time it gave a boost to Saints' hopes of SPL survival.
After a nervous opening ten minutes, Motherwell warmed to the task and in the 12th minute Mark Howard beat out a shot from David Clarkson which had been fired in from the edge of the box.
Two minutes later, John Sutton was unlucky to see a header loop wide from a Paul Quinn cross.
Motherwell continued to push forward but further efforts from Stephen Hughes and Jim O'Brien failed to trouble the keeper.
Just before the break, St Mirren almost took a shock lead. A long ball from Dorman cleared the Motherwell defence sending Craig Dargo clear, but he was denied by an excellent stop from Graeme Smith.
At the start of the second half it was the home side who continued to push forward but despite a flurry of corners they only had a weak effort from Clarkson to show for their efforts.
Mark Howard was called into action once again on 69 minutes when he dived to his left to keep out a Sutton header.
With just under 15 minutes left the home side were punished for their lack of cutting edge.
Dargo's cross into the six-yard box caused panic in the Well defence with Mark Reynolds attempted clearance falling for Thomson to score from close range.
The game was sealed for the Saints with four minutes left with a sensational strike from Dorman.
There looked to be no danger as he picked the ball up 30 yards from goal but he unleashed a stunning volley that left Smith helpless and sealed Motherwell's fate in the process.