Defeat for nearest rivals Inverness in the Highland derby at Ross County meant the three points were enough for Saints to beat Terry Butcher's men to European football.
Goals from Liam Craig and Nigel Hasselbaink secured the win against a Motherwell side who were already confirmed as SPL runners-up.
And the result meant Steve Lomas was able to deliver continental football for a second successive year after facing Turks Eskisehirspor last time.
Saints were without the suspended Frazer Wright for the clash at McDiarmid Park.
The defender was serving a two-match ban for violent conduct but Chris Millar returned from injury to bolster the side.
With number one Darren Randolph quitting in the summer for a move to Birmingham, Lee Hollis was given the nod in goal for the visitors, while Henrik Ojamaa was back from suspension.
Nicky Law, who will also depart Fir Park after this match, tried his luck early on with a hooked effort that fell just beyond the upright.
Saints thought they had grabbed the opener after seven minutes when Steven MacLean connected with Hasselbaink's pass and sent an angled drive past Hollis.
But the assistant referee's offside flag was raised before the ball hit the back of the net, cutting short the celebrations from the home crowd.
The woodwork then prevented Saints from claiming the opener when Millar pounced on a Callum Davidson cross from the left only to see his shot crash off the post and cleared for a corner.
The breakthrough finally came after 36 minutes when Craig unleashed an unstoppable 35-yard drive that left Hollis with no chance.
Motherwell could have levelled on the stroke of half-time but Alan Mannus saved well from Michael Higdon's header, before Ojamaa blasted the rebound over.
Instead, it was Saints who had the net bulging again two minutes after the restart when Hasselbaink turned and dispatched a right-foot shot past Hollis from 10 yards.
Dave Mackay then had a go with a shot from just inside the box that fell across goal and past the post.
At the other end, James McFadden could have pulled a goal back for Motherwell but his effort was weak and easily gathered by Mannus.
The result could have been beyond doubt had Murray Davidson's 20-yard volley hit the target rather than the crossbar.
Craig almost grabbed his second of the day with a powerful strike that dipped just over as the home side attempted to wrap up the points.
Gary Miller was forced out of the action late on with what appeared to be cramp, before the celebrations got under way on the final whistle as fans invaded the pitch.
- Sports & Recreation
- Nigel Hasselbaink
- Liam Craig
- Henrik Ojamaa
- Terry Butcher