Jamie Murphy's free header and a poacher's double from Scotland forward David Clarkson gave the visitors a richly deserved win and Saints were mighty relieved to hear that relegation rivals Falkirk had only drawn at Kilmarnock, despite leading at one stage.
Motherwell had to survive a couple of anxious moments after only six minutes when keeper Graham Smith smothered a half-hearted effort from Andy Dorman after the attacking midfielder had looked likely to score.
Stephen McGinn quickly latched onto the loose ball and made good contact with it, only for defender Brian McLean to make a vital goalline clearance.
The visitors slowly worked their way into the game and went ahead when Steven Hammell's swinging left-foot cross was headed low into the far corner of the net by the unmarked Murphy.
Saints responded positively but a series of poor decisions and a bad miss by John Potter at the back post stopped them getting back on level terms.
Things got even worse for them after 31 minutes as a sweet move involving Clarkson, Cillian Sheridan and Stephen Hughes allowed Clarkson to slot home his 12th goal of the season from point blank range.
St Mirren continued to have more of the ball after half-time, yet Motherwell were using their limited possession more effectively and Hughes really ought to have done better when he created a shooting chance for himself just outside the penalty box.
The Steelmen were soon on the attack again and killed off any hope of a home fightback by grabbing a third goal on the hour mark.
This time Paul Quinn crossed from the right flank and Clarkson squeezed his header under the body of goalkeeper Mark Howard.
Substitute Stephen O'Donnell pulled one back for the Buddies with a spectacular right-foot volley with just under 20 minutes left, but there was never any danger of a dramatic late comeback.