Eoin Doyle headed the hosts ahead five minutes before half-time and doubled his tally nine minutes after the break when he raced onto Paul Cairney's high ball and rocketed past Darren Randolph. Jamie Murphy tapped in from close range to cut the hosts' lead and he glanced home a leveller with 10 minutes remaining.
Hibs looked shell-shocked after losing a commanding lead and things got worse for Pat Fenlon's side when substitute Robert McHugh converted Henrik Ojamaa's cross two minutes from time.
James McPake was dismissed for a second yellow card during injury-time to complete a miserable afternoon for the home side, who now trail Motherwell by three points in the Scottish Premier League.
The hosts broke the deadlock five minutes before the break when Wotherspoon twisted away from Steven Hammell before providing an pin-point delivery for Doyle to nod home from close range.
It was Motherwell who began the second half on the front foot and Chris Humphrey should have done better after taking on Ryan McGivern inside the penalty box, but the winger could only manage a tame effort directly at Williams.
In throwing men forward Stuart McCall's side left themselves liable on the counter-attack and they were punished on 54 minutes when Cairney's through-ball created an opening for Doyle and the Irish striker finished in style for his second goal of the afternoon.
The Fir Park side continued to look for a way back into the match and they reduced the deficit when Humphrey drilled the ball low across the six-yard box and Murphy was left with a simple tap-in.
Gary Deegan made his return from injury when he replaced Griffiths with 20 minutes left and McCall showed his attacking intent as Cummins made way for McHugh.
Fenlon looked to tighten up his defence with the introduction of Ross Caldwell for Doyle, but the home side were unable to hold their lead as Higdon knocked on Hateley's dangerous corner and Murphy glanced the ball home at the back post.
McHugh then made no mistake with two minutes remaining as he turned Ojamaa's cross across the helpless Williams and into the bottom corner. McPake was dismissed with seconds remaining for a second yellow card following a tussle with Murphy.