A stunning strike from Jake Hastie sent Motherwell on their way to a fifth consecutive Ladbrokes Premiership victory as St Mirren fell further adrift at the bottom.
The 19-year-old winger netted his fourth goal in five games since returning from a loan spell at Alloa in sensational fashion in the 10th minute in Paisley.
Paul McGinn headed an equaliser in the 74th minute but Allan Campbell headed home three minutes later to earn Motherwell a 2-1 win.
St Mirren's victory at Fir Park on December 22 had cut the gap on Motherwell to six points but they have not picked up a point since, while their opponents have moved four points off the top six.
St Mirren boss Oran Kearney handed full debuts to Mateo Muzek and Duckens Nazon as six of his January signings started but Motherwell got off to the better start.
Curtis Main forced an early save before Hastie cut inside from the right wing at pace, burst past two players and thundered a left-footed strike in off the crossbar from almost 25 yards.
Kearney made a tactical switch after 22 minutes, bringing on winger Kyle McAllister for one of his three centre-backs, Jack Baird, who shook his head as he stormed into the dugout.
But the change almost paid immediate dividends as Saints created two excellent chances in the following two minutes. Mark Gillespie made a brilliant stop from Anders Dreyer from 10 yards before the on-loan Brighton player set up McAllister to fire over.
Gillespie made more routine first-half saves from Mihai Popescu and Brad Lyons while Motherwell remained dangerous counter-attacking down the flanks.
One such attack saw St Mirren outnumbered before the hour mark but Hastie's ball across the face of goal fell to a Saints player.
The hosts soon lost skipper Stephen McGinn to a muscle injury and Main twice came close to doubling Motherwell's lead.
St Mirren looked short of ideas but they levelled after the ball came back to Dreyer following his corner. The winger crossed and Paul McGinn got in front of his man to loop a header over Gillespie into the far corner.
Motherwell replied quickly with substitute Elliott Frear getting round the outside down the left flank and delivering a perfect cross for Campbell to head home unchallenged from eight yards.
Frear forced a good save with an ambitious effort as the visitors comfortably saw out the win.
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