Steven Anderson and Stevie May netted the goals in either half of Saturday's Scottish Premiership encounter as Saints continue their impressive run of form against Jackie McNamara's men.
The opening goal came on 32 minutes when David Wotherspoon sent a free-kick into the penalty area and defender Anderson powered the ball into the net at the back post.
May secured the points 14 minutes from time when he got on the end of a low Dave McKay delivery, and although Keith Watson blocked, the Saints striker quickly pounced to slip in the rebound for his 26th goal of the season.
Saints had come close to a breakthrough after just six minutes when Anderson's drive from 30 yards forced United goalkeeper Radoslaw Cierzniak into an impressive save.
The visitors failed to create anything of note until the 21st minute, when a shot from Nadir Ciftci was deflected wide of the target.
May almost doubled the lead three minutes before the break when MacLean nodded the ball down his strike partner and his first-time strike from distance came back off the post with Cierzniak beaten.
United came close a minute later when Gary Mackay-Steven drifted in behind the Saints defence and attempted to chip Alan Mannus, but the keeper got his hands to the ball.
St Johnstone were denied by the woodwork again midway through the second half when McKay cut back the ball to Wotherspoon and he thundered a shot against the inside of the post.
United have not scored against St Johnstone in the last three matches with their Tayside neighbours, and Mannus got his fingertips to keep out a downward header from Brian Graham four minutes from time.
Two minutes later, the St Johnstone keeper denied the visitors again when he got down at the feet of Ryan Dow to thwart the midfielder.