Caley Thistle had arrived in Perth top of the Premiership table on goal difference but they were undone by Tommy Wright's rampant side who simply out-passed their lethargic opponents.
But the displays from both sides were in stark contrast to their recent form. While the Highlanders had been free scoring with 16 goals before arriving in Perth, their hosts had been working off far more meagre offerings from their strikers.
Only May with his six strikes before the match had threatened with any regularity and St Johnstone's league average of just a goal a game was even tempered by the fact they netted half of their eight in a 4-0 romp against Ross County on August 17.
With those statistics in mind, it was no surprise that that victory was Saints' last in the Premiership.
Yet they started with real vigour against Terry Butcher's side and having survived an early warning when Ross Draper fired wide, they were only denied a 13th-minute opener by a marginal decision from referee Steven McLean.
Gary McDonald pounced on David Wotherspoon's low cross to the back post but Dean Brill's reactions were instant as he clawed the ball away from underneath his own crossbar. If it was over the line, it was only by a fraction but that did not stop the Saints players from making a vociferous claim.
But that was only a let-off for the visitors as Saints left no room for doubt with another May strike in the 17th minute. The 20-year-old played a ball wide for McDonald before continuing his run to the near post where he was well placed to prod home from the midfielder's cross.
The early strike seemed to give the home side a huge injection of confidence and with their passing performance polished, they created a second after 25 minutes.
Again it was a one-two combination as Mackay rolled the ball up to May on the edge of the box before the right-back darted onto the return pass and slotted underneath Brill.
Inverness forward Liam Polworth sent a free-kick high over Alan Mannus' crossbar but there was little evidence from Caley Thistle of the type of form which had seen them hit the top flight's summit. Their frustration was clear and almost boiled over as Richie Foran escaped punishment despite catching Wotherspoon with a nasty looking two-footed tackle.
Brill did well to stand up and save Nigel Hasselbaink's shot two minutes after the break but Mannus' response to Aaron Doran's rasping 30-yard effort was even more impressive as he leapt to his right to push it over.
May was denied a second by another Brill save as MacLean and Wotherspoon combined brilliantly before the latter rolled the ball back for the striker to hit. His desire for another goal was again evident in the 67th minute when MacLean sent him clear on the left and he cut inside and fired just wide of the far post.
However, the Scotland Under-21 cap turned provider for a second time as he teed up MacLean to put the win beyond doubt with a third in the 73rd minute. Once more it was an exchange of passes which left Inverness twisting in the wind as MacLean delayed his strike before planting the ball into Brill's bottom-right corner.
And May claimed his third assist with nine minutes remaining as he muscled his way to the byline before picking out MacLean unmarked at the back post to slide home his fifth of the season.