Goals from Callum Booth and Stevie Lawless mean Thistle are now up to seventh and just two points behind Saints with two games in hand.
However, it was the manner of the defeat that will alarm the Perth followers as St Johnstone were taken apart by Partick at times.
Although Chris Kane scored in 67 minutes it gave false hope as they never looked like recovering the first half damage.
Normally so well organised and switched on they allowed the Maryhill men so much room in midfield and although they were tighter in the second half, it was a poor performance from a team with just two wins in the last 15 games.
Stuart Bannigan could not believe his luck as he dominated the game with Sean Welsh and Stevie Lawless and it was he who set up the opening goal in six minutes.
He raced from his own half played a wall pass with David Amoo then switched the play to the left side where Callum Booth had all the time in the world to cut onto hhis right foot and slam a low shot into the right corner.
Remarkably, having not scored in any of his first 41 matches for Thistle, it was his second successive goal after scoring in last Saturday’s 3-1 defeat by Motherwell.
Saints were punished again in 13 minutes when Lawless had so much freedom on the edge of the box after taking pass from Mathias Pogba and drilled a left foot into the right corner.
Tommy Wright looked ashen-faced on the touchline and it did not get much better for the Saints manager as Alan Mannus had to make a smart save from Sean Welsh’s free kick in 19 minutes.
Then the Northern Irish keeper made an even better block on Lawless’s shot in 34 minutes, somehow pushing it over the bar.
To add to Wright’s misery on the one occasion his side threatened in 23 minutes Graham Cummins made a mess of the chance, dragging the ball two wide when he was played in by Danny Swanson and missing the target.
Inevitably there was a reaction from the Perth side in the early moments of the second half no doubt induced by some words of encouragement from their manager.
However, they might have been dead and buried had Welsh not headed Bannigan’s cross straight into the arms of Mannus in 62 minutes.
In fairness Saints managed to claw themselves back into the game five minutes later when substitute Chris Kane came in at the back post to bundle Darnell Fisher’s looping cross over the line.
However it papered over the cracks of a dismal performance and it was Thistle who finished the game stronger.
Welsh shot weakly at Mannus in 80 minutes when he should have done better then substitute Kris Doolan was denied in 86 minutes by a terrific Joe Shaughnessy block.
Referee: C Thomson
St Johnstone: Mannus; Fisher, Shaugnessy, Anderson, Scobbie; Wotherspoon (Thomson 77), Davidson, Craig, Swanson; Cummins (Kane 45), MacLean (Sutton 82) Subs not used: Clark, Lappin, Doyle, Krachunov.
Partick Thistle: Cerny; Miller, Lindsay, Seaborne, Booth; Osman, Bannigan; Amoo (Elliott 65), Welsh, Lawless (Edwards, 90); Pogba (Doolan 86) Subs not used: Scully, German, Nesbitt, Nisbet.