Under-20s coaches Duncan Shearer and Scott Kellacher took charge of the team, with Caley still looking to replace Terry Butcher, and they were buoyed by a solid display that saw them stifle almost all the Saints positive play.
It took the home side less than four minutes to get their noses in front as Carl Tremarco beat two attempted tackles down the left and whipped in a lovely ball begging for McKay to tap home.
The visitors struggled to create chances, a shot wide from Nigel Hasselbaink a rare threat, and McKay came close to doubling his tally when he forced Alan Mannus into a decent save on the half-hour mark.
David Wotherspoon almost brought St Johnstone level before the break but he was denied by a brilliant block by Tremarco, and Caley went straight down the other end only to see McKay just fail to link up with Ross Draper.
Caley's dominance continued in the early stages of the second period and Draper should really have done better than shoot over from 10 yards a couple of minutes in.
Stevie May received some dark looks from Saints team-mates on the hour when he greedily shot at goal when others were far better placed.
They became increasingly frustrated in their ventures forward with Caley happy to keep the ball in midfield, a long way from Dean Brill's goal.
And their frustrations would continue to the final whistle with Tom Scobbie going into the book as injury-time wound down after fronting up to the referee, no doubt complaining at Caley's time-wasting tactics.