Two goals in three first-half minutes from Michael Higdon and Nicky Law put Motherwell in command and Jamie Murphy scored the pick of their goals with a volley just after the interval.
Liam Craig pulled one back from the spot but substitute Henrik Ojamaa netted two late goals on the break as Motherwell moved five points clear in third place in the SPL.
A late equaliser for Hearts against Dundee United in the lunchtime kick-off had prevented Motherwell dropping out of the top three for only the second time this season and they capitalised on their closest rivals' slip-up emphatically.
The win moved Motherwell eight points above St Johnstone and nine above Hearts and, with both of those clubs playing each other in the final three games, Motherwell cannot finish outside the top five.
Third place is well within their grasp too with their front players, especially Chris Humphrey, finding form. They also benefited from the restoration of the midfield partnership of Steve Jennings and Keith Lasley following recent suspensions to record their first win in five games.
St Johnstone were without Cillian Sheridan as well as his suspended strike partner Francisco Sandaza and they never recovered from the two-goal burst.
Murphy was restored to the side in a forward position and curled just wide of the far corner from 20 yards in the fifth minute but St Johnstone edged the opening exchanges.
With Marcus Haber up front alone, their midfielders were getting the chance to run at the home defence and Darren Randolph saved decent long-range efforts from David Robertson and Derek Riordan.
The key first-half moment came when Motherwell were awarded a disputed penalty after Steven Anderson was adjudged to have brought down Humphrey just inside the box.
Humphrey had got away from Craig, who failed to bring him down, and ran at Anderson, who slipped as Humphrey put the ball past him before falling over his body.
Although it did not look deliberate, Humphrey was impeded and referee Bobby Madden was well-placed to point to the spot before Higdon sent Alan Mannus the wrong way.
Humphrey created Motherwell's second three minutes later with some good skill on the bye-line before driving the ball across the face of goal for Law to slide home from close range. Riordan curled a free-kick just wide but Saints were struggling to cope with Motherwell's attacks and Humphrey in particular.
The right winger saw another dangerous low cross cut out before meeting a ball from the opposite flank and drawing an excellent one-handed save from Mannus, who held Murphy's follow-up.
Murphy then found space down the left before cutting inside to pass to Higdon, whose flicked effort on the stretch from 18 yards was touched just wide by Mannus.
Randolph saved another long-range Riordan strike and then came off his line quickly to block from Robertson before the break.
The home side extended their lead seven minutes after the restart with an impressive finish from Murphy.
Tom Hateley dug out a cross from the right which the former Scotland Under-21 forward powerfully volleyed into the corner of the net from 16 yards.
Murphy soon had a great chance for number four when David McCracken missed Randolph's kick-out but the forward's attempted lob lacked conviction and Mannus pushed the ball away.
Saints got a lifeline on the hour mark when Madden ruled Shaun Hutchinson had fouled Haber as the striker tried to turn the defender inside the box. Craig sent Randolph the wrong way.
Mannus saved Higdon's shot on the turn and Murphy's curler as Motherwell set out to finish the game off.
Saints lacked ideas going forward and Ojamaa finished the game off in the 84th minute after another assist from Humphrey.
The Jamaican-born wide man sprinted away from Jody Morris and picked out Ojamaa in the box with the Estonian taking a touch before slotting the ball into the bottom corner.
Ojamaa grabbed another three minutes later when he cut in from the left and fired the ball under the body of Mannus, who could not prevent the ball crossing the line as the assistant flagged for the goal.
