Ian McCall's opener as United's manager brought their first victory at home since April in this basement battle.
His team had a dream start in the fourth minute when Charlie Miller's return came to Andy Tod for the home debutant to smack a 15-yard shot low giving Stephen Woods no chance.
The Tangerines, in full cry, really should have extended their lead but for two glaring misses. A wonderful flick on from Tod in the seventh minute following Gary Bollan's free-kick gave Allan Smart a clear opening but he miscued.
Then the same player wasted an even better chance two minutes later when Miller sent him clean through. Corrigan's last gasp tackle saved the day.
The dream turned sour for Tod in the 17th minute. A weak back pass gave Clarkson an unexpected present. As he rounded Alan Coombe, the keeper brought him down and gave the referee no choice but to produce a red card.
Substitute Paul Gallacher's first touch was to then pick the ball out of the net from Derek Adams rifled shot.
This rattled United who, having looked so commanding began to be pulled around the park as the visitors grew in stature following their good fortune.
The second half began as the first ended with Motherwell exerting the pressure and the referee waved away a strong penalty claim from United for handball.
Captain Scott Leitch had the opportunity to edge the visitors in front as he eased past Tod but could not manage to hit target. While at the other end Woods pulled off a spectacular stop from Lilley's bullet header.
In the 69th minute Charlie Miller's first goal of the season won the game. Stuart Duff who had toiled on the left midfield, somehow managed to drill the ball across and Miller was first to react, toe poking the ball high into the net past the flaying arms of Woods.