Goals from Gary Hooper and Efe Ambrose, then Victor Wanyama's double, gave an under-strength Hoops side an unassailable interval lead against sorry-looking Saints.
Tony Watt scored with four minutes remaining to seal a commanding victory, which manager Neil Lennon would have wanted ahead of its UEFA Champions League Group G clash with Barcelona on Tuesday night (Wednesday AEDT).
St Johnstone lifted itself up to second place with a fifth consecutive top-flight win for the first time since 1971.
Murray Davidson set the Perth side on its way against Kilmarnock with a scrappy opener in the 29th minute, and it stayed that way until a minute from time when Nigel Hasselbaink netted a second.
With the game now beyond it, Kilmarnock scored what proved to be no more than a consolation in injury time courtesy of James Fowler's deflected strike to make it 2-1.
A late goal from Colin McMenamin, his first of the season, was enough to earn Ross County its first home victory in the SPL, 3-2 over Hibernian.
In an entertaining match, Leigh Griffiths gave Hibs the lead in the sixth minute but Stuart Kettlewell levelled three minutes later.
Iain Vigurs then gave County the advantage in the 34th minute before Hibs equalised through James McPake on the stroke of half time.
The match looked to be heading for a draw in an equally entertaining second-half but super sub McMenamin popped up with the winner seven minutes from time.
Niall McGinn was on target as Aberdeen extended its unbeaten run to 11 games with a 1-1 draw against Dundee United that left the host winless in six.
John Rankin's 22nd-minute goal was cancelled out eight minutes into the second-half by McGinn, who netted in Northern Ireland's midweek draw in Portugal on the occasion of Cristiano Ronaldo's 100th cap.