The visitors’ Aidan Nesbitt inadvertently clipped the heels of Keith Lasley as the Motherwell skipper ran on to Steven Hammell’s return pass inside the penalty box.
Louis Moult converted the resulting 37th-minute spot-kick and the hosts soon extended their lead when Moult ran on to Scott McDonald’s reverse pass and finished well. Marvin Johnson broke away to fire a third in the 69th minute.
Thistle briefly rallied after Connor Ripley spilled Callum Booth’s swerving effort inside his near post but the hosts were well worthy of their first three Ladbrokes Premiership points of 2016 at Fir Park.
Motherwell manager Mark McGhee changed to a 4-3-3 formation with Johnson joining McDonald and Moult in a fluid three-pronged attack.
Moult, who took his tally for the season to 16, said: “I think we linked up really well at times.
“If it was Scotty and Marv linking up and me getting on the end of it, or vice-versa, it definitely worked.
“Myself, Marvin and Scotty are all different. We bring different things to the game and I think it worked.”