Erwin was on target twice inside 25 minutes as the hosts made it three straight wins to boost their hopes of remaining in the Scottish Premiership next season.
The result leaves Saints 10 points adrift at the base of the table and, with just six games remaining, their chances of avoiding the drop appear slim.
Erwin needed just seven minutes to open the scoring, converting Lionel Ainsworth’s low cross to fire the hosts ahead, as he found the bottom right-hand corner of the net from 12 yards.
Ainsworth was also the provider for the second goal 18 minutes later when Erwin’s turn and shot located the opposite corner of the net.
The third Motherwell goal arrived on the stroke of half-time when Ainsworth’s deep cross to the far post was met by Marvin Johnson, who laid the ball into the path of scorer Scott McDonald.
Steven Thompson did his best to reduce the arrears after the break. First, he fired narrowly wide of the left post on the hour mark before having a close-range header saved by keeper George Long.
However, Motherwell extended their lead on 74 minutes as Johnson’s deep cross was met by substitute Sutton, who dived to head home from the edge of the six-yard box.
And if St Mirren needed any confirmation it was not to be their night, Stephen Mallan struck a post from 12 yards before Sutton completed the scoring 11 minutes from time.
The striker pounced on a loose ball to slide in the fifth from close range after McDonald’s initial effort had been blocked.
However, with Ross County beating St Johnstone by a single goal, Motherwell remain second from bottom in the table despite making it 13 points from the last 18.