Liam Gordon scored the all-important winner as he nodded home from close range after just 11 minutes.
From there on in the visitors were content to hold onto their lead through doing the basics and employing effective game management strategies – much to the annoyance of the home support.
A straight red card for Killie starlet David Watson after a full-blooded challenge at real speed on Cameron Ballantyne gave the hosts a mountain to climb.
John Beaton’s decision was backed by VAR with no trip to the pitchside monitor required after a viewing by Andrew Dallas.
And St Johnstone didn’t really look like giving up their lead with Steven MacLean’s men well-drilled in their press, resolute in defence and clever in their running down of the clock when it was necessary.
Danny Armstrong went close with a drive from range for the hosts but St Johnstone should have doubled their lead with Graham Carey and Zak Rudden going close.
There was a final nervy moment in the dying seconds with a VAR check for a possible penalty after Vassell went down in the box, but all that was left was for Beaton to blow for full-time. Cue wild St Johnstone celebrations on the pitch and in the stands.
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Battle at the bottom
Just three points now seperate the bottom three clubs with two matchdays to go.
St Johnstone have hauled themself safe leaving a shootout between Dundee United on 31 points, Ross County on 33 and Kilmarnock on 34.
All three sides lost on Saturday leaving a blockbuster final few matches remaining in the bottom half of the split.
Kilmarnock take on Dundee United next time out before meeting Ross County on the final day of the season.
A dramatic climax is guaranteed which can only be good for Scottish football – even if Jim Goodwin, Derek McInnes and Malky Mackay would rather anything else.
Blunt Kilmarnock struggle
It’s almost expected that affairs between sides playing for their future in the division would be typically cagey.
Neither side wanting to make a mistake but equally desperate for three points.
Unfortunately for the hosts, even with time dwindling, they were unable to take control and mount significant pressure on their opposition.
Derek McInnes’ men looked lost when the ball headed towards the St Johnstone goal with passes all too often going backwards with a real lack of incision in the forward areas.
Kyle Vassell and Christian Doidge were largely found flicking balls on without the necessary support from midfield – only worsened when Watson trudged off after being dismissed following a largely positive contribution to that point.
If Kilmarnock are to drag themselves clear of the drop, a scoring touch is badly needed.
St Johnstone grind it out
When Scottish Premiership safety is at stake, it doesn’t matter how it is achieved and there certainly aren’t any bonus points for flair.
St Johnstone grabbed the match by the scruff of the neck in the opening exchanges and when they got their goal it wasn’t something they would give up easily.
No risks at either end of the pitch but a wholly committed effort with a lung-busting press throughout and little to trouble Remi Matthews in goal.
Just as powerful as Gordon’s opener in deflating the hosts was clever game management from the Perth outfit; be it Matthews collecting a second ball for goal kicks, Drey Wright losing his boot or Gordon’s shinpads inexplicably falling out leading to a delay as he subbed them out.
The home crowd were understandably irked, the visiting support were loving it. It’s a part of the game often criticised but most importantly, it got the job done.