Goals by Devante Cole, Peter Hartley, Jake Carroll and Christy Manzinga did the damage and cemented 'Well's fourth placed position after 15 games.
St Johnstone looked the sharper side in the early exchanges but didn't create any major chances of note.
It was then better from the Steelmen as Jermaine Hylton shoeds skill and found Chris Long but his shot did not trouble Clark.
The Steelmen created an opening on 24 minutes when a bulldozing run by Liam Grimshaw set up Long, who displayed sharpness to make space but shot wide from outside the box.
Hylton then made space for himself but blasted a 26th minute effort narrowly off target.
Motherwell went ahead on 28 minutes when fine wing play by Hylton on the left set up Cole who headed in from close range.
Saints' David Wotherspoon produced a fine run down the left flank on 38 minutes and his subsequent cross picked out Chris Kane whose backheel flick was nifty but straight at Gillespie.
Two minutes before half-time the hosts almost doubled their lead when Polworth's corner from the right was met by an unmarked Hartley whose powerful header was saved by keeper Zander Clark.
The opening chance of the second half saw a Jake Carroll cross from the left headed well over by Cole.
It was 2-0 the Steelmen on 50 minutes when Polworth's fine corner from the left was bulleted in by the head of Hartley.
Cole spurned a great opportunity just before the hour mark when he advanced to the edge of the box and skewed a shot wide when a pass right would have presented an unmarked Long with a great chance.
Tanser's cross from the left was headed over by Alistair McCann as Saints hit back before a Stevie May cross was turned wide by Kane.
Hylton was involved again on 70 minutes as his shot forced Clark into a smart save.
The killer third goal arrived on 72 minutes when Carroll netted the rebound after Polworth's corner saw a Gallagher header saved.
Motherwell rounded it off in the final minute when substitute Manzinga slotted the ball home after James Scott's long ball had been headed down by Mark O'Hara.
Motherwell: Gillespie, Carroll, Grimshaw, Hartley, Campbell, Long (Scott 61), Hylton (Manzinga 77), Polworth (O'Hara 79), Donnelly, Gallagher, Cole.
Referee: Euan Anderson