The 2-1 Fir Park victory – played in a lively atmosphere – was deserved for ’Well as they all but eliminated their relegation fears and pushed bottom club United nearer the trap door.
United started strongly and Paul Dixon signalled their early intent with a left footed effort into the side netting.
Motherwell somehow failed to go ahead three minutes later when United keeper Michal Szromnik miraculously saved Scott McDonald’s close range effort after Chris Cadden’s cross from the right.
McDonald had the ball in the net six minutes later, but the effort was ruled offside.
Ex-Motherwell striker Henri Anier suffered the same fate at the other end before he did put United ahead in stunning fashion after 21 minutes.
Anier superbly struck the ball into the top corner of Connor Ripley’s net after a pass inside by John Rankin.
Dixon’s 30-yarder was then well stopped by Ripley before ’Well’s Stephen Pearson left the field with a rib injury on 34 minutes to be replaced by Morgaro Gomis.
Ripley produced a fine stop from Billy McKay before ’Well equalised just before half-time when Josh Law’s free-kick looped into the net off Johnson’s head.
The goal seemed to give Johnson an incredible lift as he was fantastic early in the second half with fine runs and crosses. His left footed effort on 55 minutes was deflected onto the bar and over.
There was a major blow for ’Well three minutes later when influential skipper Keith Lasley limped off injured after a crude looking challenge by Callum Morris as the players went in for a 50-50 ball.
Lasley was replaced by Dom Thomas.
’Well winner arrived on 65 minutes when McDonald’s pass found Johnson, whose inch perfect cross from the left found Moult who tapped in gleefully for his 17th goal of the season.
Cadden then shot straight at Szromnik from a Moult pass after an incisive breakaway before United – who had been combative throughout – incurred bookings for Morris’s foul on Cadden and Gavin Gunning’s kick at Gomis.
Law was the third ’Well man to go off injured when he was replaced by Kieran Kennedy on 86 minutes.
The final goal attempt of a rain drenched evening in injury time saw man-of-the-match Johnson’s effort deflected wide after a fine run.