McLean grabbed the only goal of the game late on to provide one of the few highlights in what was a largely dull affair at McDiarmid Park.
The win allowed the Paisley club to bounce back from their midweek Scottish Communities League Cup exit to First Division side Ayr United.
St Johnstone were boosted by the return of Cillian Sheridan, who recovered from a hamstring injury to start ahead of Marcus Haber as the Perth side made one change from the goalless draw against Dundee United.
St Mirren made two changes from the side who lost to Ayr, with Jon McShane and Graham Carey both added to the team.
There was little in the way of goalmouth action early on as both sides struggled to impose themselves upon the game.
St Johnstone eventually had the chance to put their visitors under pressure when Carey handled 25 yards out but Dave Mackay's deflected free-kick was comfortably smothered by goalkeeper Craig Samson.
Kevin Moon then resorted to a long-range effort that he ballooned well over the crossbar, with Samson untroubled on this occasion.
At the other end, Carey saw a free-kick of his own flash across goal and past the post, before an effort from McLean was held by goalkeeper Peter Enckelman.
Francisco Sandaza then called Samson into action, who blocked well from the close-range shot at the near post.
The home side were threatening again when Sheridan tried his luck with a left-foot shot that bobbled awkwardly on the turf but was saved by the goalkeeper.
McLean had a great chance to open the scoring three minutes after the restart when he embarked on a decent run into the box before unleashing the shot which was pushed one-handed over the crossbar by Enckelman.
Alan Maybury - who replaced Frazer Wright at half-time - then teed up a shot for Sandaza but he fired inches past the upright for the home side.
Samson came to the rescue for the Buddies with an impressive save to deny Sheridan after a Liam Craig corner broke to him just inside the box and his shot looked set to leave the net bulging.
Substitute Murray Davidson won a free-kick for St Johnstone and Craig picked out Sandaza in front of goal only for the Spaniard to nod just wide.
Instead it was St Mirren who finally made the breakthrough when they opened the scoring with 81 minutes on the clock.
Steven Thompson nodded on for McLean and he left Enckelman with no chance as he rifled into the back of the net from 15 yards.