Scorers: St Johnstone - MacLean 6′, 35′ Miller 87′
MacLean, who was returning from knee surgery, grabbed his first of the evening when goalkeeper Gunnar Nielsen parried a ball into his path after saving a Stevie May free-kick.
He then scored his and the Saints’ second when Brian Easton’s cross from deep on the left found him nipping in ahead of Shaun Hutchinson at the back post in a first half dominated by the home side.
The points were secured for the Perth side late on when Gary Miller ran down the left unchallenged before cutting inside and lashing an unstoppable shot into the net.
MacLean’s inclusion was one of four changes following St Johnstone’s home defeat against Inverness Caledonian Thistle on Saturday, with Lee Croft, James Dunne and Miller all coming in, while Craig Reid made his first start for Motherwell and manager Stuart McCall also gave a start to Henri Anier.
Motherwell created the first real chance of the game when Anier spun away from Steven Anderson on the edge of the area and hit a first-time shot that sailed just wide.
It was St Johnstone however that took the lead after six minutes.
May took aim 30 yards from goal and his free-kick should have been better dealt with by Nielsen, but he made a mess of attempts to gather and the ball ran straight into the path of MacLean who could not miss from four yards out.
Wonderful play by May on the right then saw him cut a great ball back for the onrushing Nigel Hasselbaink but his sliding attempt to connect was cut out well by Shaun Hutchinson.
Motherwell were contributing very little to the game but Iain Vigurs brought out a fingertip save from Alan Mannus in the St Johnstone goal to deny the former Ross County man.
St Johnstone extended their lead after 34 minutes with MacLean again on target.
Easton’s delivery ran through to MacLean who somehow got in front of Hutchinson to poke home his second.
The visitors should have pulled a goal back on the stroke of half-time when a free-kick from Vigurs found its way to the unmarked Stephen McManus at the back post.
He contrived to skew the ball wide from four yards, but his incredible miss meant St Johnstone went into the interval 2-0 up.
Clearly upset with his side’s first-half showing, Motherwell manager Stuart McCall threw on former Scotland forward James McFadden and Lionel Ainsworth for Stevie Hammell and Vigurs in an attempt to salvage something from the game.
Again, though, it was St Johnstone who started the brighter and Nielsen had to look smart to beat away a fierce 25-yard drive from Lee Croft.
Nielsen looked to be trying to atone for his first-half error, and he got down well to fingertip away a May drive from the edge of the area, and then made another flying save to deny David Wotherspoon.
Two-goal MacLean was replaced by former Scotland striker Chris Iwelumo with 19 minutes left, and within seconds of coming on he missed a glorious chance to put Motherwell out of sight.
Terrific wing play from fellow substitute Wotherspoon found the striker unmarked inside the six-yard area and somehow he managed to shoot straight at the goalkeeper rather than burst the net with his shot.
Four minutes from time, Miller secured the points for St Johnstone after bursting 50 yards down the left before cutting in and under no pressure firing the ball home.
The result saw the Perth side move three points ahead of seventh-placed Hibernian in the race to finish in the top six ahead of their trip north to face in-form Aberdeen at Pittodrie on Saturday.
St Johnstone: Mannus, Miller, Mackay, Anderson,B Easton, Croft (Millar 81), McDonald, Dunne, Hasselbaink (Wotherspoon 64), May, MacLean (Iwelumo 72). Subs not used: Cregg, Banks, O’Halloran, Gilchrist.
Motherwell: Nielsen, Reid, Hutchinson, McManus, Hammell (McFadden 46), Anier (Shirkie 80), Lasley, Carswell, Vigurs (Ainsworth 46), Francis-Angol, Sutton. Subs not used: Hollis, Murray, Leitch, Cadden.