St Mirren were beaten at The SMiSA Stadium with Bozo Mikulić's goal the difference on a disappointing afternoon in Paisley.
After a brilliant win away to Aberdeen last weekend, manager Stephen Robinson made just one change to the side with Elvis Bwomono coming in for Callum Penman.
The Buddies started brightly with Mikael Mandron racing clear on goal with just five minutes on the clock. However, the forward's tame effort was saved by Ross Sinclair with the offside flag raised.
Caolan Boyd-Munce sent a free-kick off the wall from 20 yards just before the half-hour mark, while Mandron shot into the arms of Sinclair from just inside the area.
VAR checked for a St Mirren penalty on 34 minutes when Bwomono went down under a challenge in the box. But nothing was given to the dismay of the Saints full-back who looked to have been clipped by Drey Wright. Mark O'Hara was next to have a go for the Buddies but his strike from 20 yards was gathered by Sinclair as the game headed into half-time goalless.
Zach Hemming was first called into action on the hour when he got two strong hands behind a Graham Carey strike with the Saints stopper up quickly to then deny Makenzie Kirk on the follow up.
St Mirren thought they'd finally made the breakthrough on 68 minutes when substitute Greg Kiltie finished from close-range. But referee Dan McFarlane disallowed the effort for a Toyosi Olusanya foul in the build-up.
How frustrating costly that moment would prove to be when the visitors took the lead three minutes later. Saints couldn't deal with the loose ball after a corner and that allowed Mikulić to knock home from close range to score the only goal of the game.
Full-Time: St Mirren 0-1 St Johnstone
St Mirren: Hemming, Fraser, Gogic, Taylor, Bwomono (Oseni 82), Phillips, Boyd-Munce (Smyth 82), O'Hara (c) (Kiltie 63), Tanser, Olusanya (Mooney 82), Mandron (Scott 63)
Subs Not Used: Urminsky, Kenny, John, Penman
St Johnstone: Sinclair, Sprangler (c), Mikulić, Douglas, Curtis, Holt, Watt, Wright, Carey (Steven 74), Griffith, Kirk (Sidibeh 74)
Subs Not Used: Rae, Hepburn, Smith, Franczak, Kucheriayvi, McPake, Clark
Referee: Dan McFarlane
Assistant Referee: Calum Spence
Assistant Referee: Alastair Mather
Fourth Official: Euan Anderson
VAR: Alan Muir
Attendance: 6,298
