The winning ball also gave the Steelmen added confidence ahead of Thursday's second leg UEFA cup tie with Nancy.
The game got off to a flying start with the sides trading chances in the opening ten minutes. St Mirren were first to threaten when Andy Dorman and Dennis Wyness combined to set up Billy Mehmet, but the striker's poor poke was saved by Graeme Smith.
At the other end a John Sutton header brought an excellent save from Mark Howard. Then back at the other end Wyness hit a stinging angled drive which was well saved by Smith.
It was therefore a bit of a surprise when it took half an hour before they got the opening goal. It came when Steven Robb beat Keith Lasley on the left and his cross picked out Mehmet, who took his time to bend the shot past Smith.
Motherwell's response to going behind came from a Bob Malcolm free-kick from 20 yards out that was tipped onto the crossbar by Howard.
The home side made the best possible start to the second half by equalising after only two minutes. Steven Hammell's corner was missed by Saints keeper Howard, with Sutton planting a free header into the net.
Fourteen minutes later they fashioned a second to take an unlikely lead. Jamie Murphy took a short corner to Hammell and with the keeper flapping again, Malcolm was on hand to bundle the ball over the line.
Both sides then had long-range efforts with Franco Miranda seeing a 25-yard shot saved by Smith, while at the other end a spectacular Hammell volley crashed clear off the crossbar.
Things got worse for Saints with 20 minutes left when Miranda was shown a straight red card for what the referee's assistant saw as a shove on Sutton.
This allowed Motherwell to hold on and secure a vital three points.