Motherwell won for the fifth time in six matches as goals from James Scott and Chris Long saw off a St Mirren side who have now failed to score in their last three games.
Stephen Robinson's side dominated proceedings and only some terrific saves from goalkeeper Vaclav Hladky prevented the home side from adding to the 2-0 victory margin.
St Mirren came closest from a Paul McGinn stab at goal that was cleared off the line.
Motherwell had made one enforced change, with Barry Maguire in for the suspended Liam Donnelly.
St Mirren left Tony Andreu and Junior Morias on the bench, bringing in Danny Mullen and Jon Obika.
Obika spurned an early half-chance for the visitors but after that it was all Motherwell.
First, Scott darted across the edge of the penalty box but dragged his shot wide. Sherwin Seedorf then set up Liam Polworth who pulled out a good save from Vaclav Hladky, before the Dutchman's dribble and shot was also repelled.
Defender Declan Gallagher then got in on the act, advancing to the goal line before cutting the ball back for Devante Cole. Again Hladky kept the effort out.
Motherwell then spurned a great chance on the counter following a rare St Mirren corner. Scott's pass to the advancing Polworth was weak, forcing the former Inverness man into a long-range shot that went wide.
The home side continued to pile on the pressure and the impressive Polworth had another shot saved by Hladky, who also then denied Cole for a second time.
The breakthrough was coming, however, and duly arrived after 39 minutes. It was a terrific strike, Scott taking a pass from Maguire and firing it into the top corner. It was no more than Motherwell deserved.
St Mirren started the second half with a bit more energy and Motherwell goalkeeper Mark Gillespie was finally called into action when he pushed over Mullen's angled effort.
From the resulting corner the loose ball fell to McGinn, whose stab goalwards was hacked off the line.
Sam Foley then tried to take matters into his hands with a lung-bursting run from midfield but his shot at the end was weak, before Kyle Magennis' shot was headed away by Peter Hartley.
St Mirren were belated showing some life and Ryan Flynn thundered a shot over after good work by Calum Waters.
Long made sure of the points with Motherwell's second goal four minutes from time, cutting inside before firing high past Hladky.
