It took only 65 seconds for the Saints to take the lead when Jason Scotland out paced the Thistle defence to latch on to Steve Milne's pass and fire confidently pass Kenny Arthur.
Thistle did not learn from this early set back and went further behind after six minutes when, following a good exchange between William McLaren and Martin Hardie, Scotland had an easy tap in from six yards.
The Saints were rampant and Thistle had no response and could have gone three down on 19 minutes but for Arthur's save from Milne's 14-yard shot.
As the game progressed, Thistle did manage to put some passes together but did not present and difficulties for the St Johnstone keeper and it came as no surprise when the visitors extended their lead on the stroke of half-time.
When Milne beat Arthur to a Hardie pass he was able to chip the ball over the keeper and into the empty net.
In the second half Thistle were unable to change the pace of the game and after 64 minutes the Saints put the game out of their reach when Milne scored his second with a 16-yard shot after Arthur could only parry a close range shot from Scotland.
Thistle did get a consolation on 72 minutes when Jimmy Gibson was fouled in the box and Simon Donnelly made no mistake from the spot.
Despite this goal Thistle did not have any further chances to reduce the deficit and deep into injury time Scotland completed his hat-trick when he took advantage of sloppy defending to score from close range.