Thistle had taken the lead with an early Steven Lawless effort before Keith Lasley, Zaine Francis-Angol and James McFadden turned the game in an 18-minute spell.
Lionel Ainsworth and John Sutton added further goals after the break as the visitors ran out comfortable winners.
Former Motherwell winger Lawless put the Jags ahead after just eight minutes when he steered the ball into the bottom corner after Kris Doolan and James Craigen combined to create the chance.
But the Lanarkshire side hit back when McFadden took the ball in from a throw-in, turned his man and slipped the ball into the path of Lasley (15) to place the ball in the far corner.
Six minutes later Steven Hammell found Francis-Angol and he cut inside Stephen O’Donnell to shoot past Scott Fox at his near post.
Thistle were breeched with consummate ease again on 36 minutes when Henri Anier pounced on a loose ball and lofted it forward for McFadden, who held off a challenge from Gabriel and finished with a lob over the advancing Fox.
Motherwell increased their advantage 12 minutes after the break when Aaron Taylor-Sinclair gifted possession to Lasley, who played in Ainsworth to slot the ball into the net.
McFadden powered a header off the bar shortly before Sutton (65) ran on to a long pass from Iain Vigurs and steered the ball home.
And McFadden played Ainsworth through two minutes later but Fox came off his line to block the shot and the former Everton player curled just wide from 25 yards as Motherwell threatened to add to their tally.