Steve Lomas' side are unbeaten in five games after an indifferent start to the season and were far too strong for a disjointed Dundee side at Dens Park on Saturday.
First half goals from Gregory Tade and Liam Craig allied to Steven MacLean's second-half breakthrough meant Ryan Conroy's brief equaliser mattered little.
The on-form Perth side occupied bottom place in the league earlier this month but have kicked into gear in some style recently.
But for Dundee there seems no end to their suffering as they again failed to record their first home league win since returning to the SPL.
MacLean turned Dundee down during the summer in favour of a move to Saints - and showed them what they were missing as he completed the rout to keep the McDiarmid Park bandwagon rolling.
There was no greater incentive for the hosts than having the opportunity to break their proverbial home hoodoo with what was now the fourth attempt to do so in front of their own supporters.
And they ought to have taken the lead in the fifth minute when Jim McAlister found himself bearing down on the St Johnstone goal.
Latching onto Ryan Conroy's through-ball, McAlister went one-on-one with Saints goalkeeper Alan Mannus but saw the goalkeeper deflect his effort wide for a corner.
Dundee's bright beginning continued when Colin Nish pounced on Steven Anderson's short header back but again Mannus was alert enough to come rushing from his line and gather.
But, in 16 minutes, they were made to pay when Saints opened the scoring with their first real attack of the match.
Gary Miller started things off with a pass which beat the Dundee offside trap for the onrushing Liam Craig.
With his weaker right foot, Craig crossed for Tade who duly swept the ball high into the net past the despairing goalkeeper Rab Douglas from 10 yards.
It was tough on the hosts but they refused to buckle and got themselves back on level terms in the 22nd minute from the penalty spot.
Nish's headed flick-on caused panic in the Saints defence where Frazer Wright was adjudged to have connected with a high boot on debutant Martin Boyle.
Referee Alan Muir pointed to the penalty sport without hesitation and Conroy calmly made it 1-1 with a precise penalty which sent Mannus the wrong way.
However, Saints regained the lead, helped by some more shoddy Dundee defending, in the 27th minute.
Miller again broke clear down the right and his whipped cross eluded Tade but Perth team-mate Craig ghosted in at the far post to send a fine finish past Douglas from the tightest of angles.
In the 52nd minute, MacLean made it 3-1 to the visitors with a close-range finish after Wright had a free header to set him up from Craig's corner.
Saints could easily have increased their lead before the end had it not been for Douglas in the Dundee
goal.
It is clear this season is going to be nothing other than one almighty slog to stay up for the SPL new boys - who are at Hibernian next weekend.
