Andrew Shinnie opened the scoring for the hosts nine minutes into the second half and McKay wrapped up victory with goals in the 61st and 78th minutes.
Inverness manager Terry Butcher enjoyed some cheers from the home fans who showed appreciation for him turning down the chance to move to Championship side Barnsley earlier this month. His commitment to the Highlanders came as his men extended their unbeaten run to 10 games in all competitions
Caley Thistle keeper Antonio Reguero was called into action early, tipping over an effort from Aberdeen's top scorer Niall McGinn. At the opposite end Richie Foran tested keeper Jamie Langfield, who turned the chance wide.
Former Caley Thistle star Jonny Hayes then fired over in an end-to-end opening period. Hayes had another chance at the mid-point of the first half but Reguero was equal to his low shot.
Andrew Shinnie was unmarked as he guided a header inches over from a David Raven cross. Hayes continued to threaten on the right wing and cut the ball back to McGinn who shot just over. Joe Shaughnessy also curled an attempt wide as neither side could be separated at the break.
Aberdeen started the second half brightly with Hayes cutting inside Graeme Shinnie and Gary Warren before being thwarted by a terrific save from Reguero. It proved crucial as Inverness made the breakthrough through Andrew Shinnie on 54 minutes.
After being played in by Aaron Doran, he cut inside Dons captain Russell Anderson then rifled the ball past Langfield into the roof of the net from an acute angle. It was the Scotland attacking midfielder's 12th goal of the season.
Seven minutes later Mckay doubled Inverness's lead. The SPL's top scorer showed composure to tuck the ball past Langfield from 10 yards after being found by Ross Draper. Midfielder Draper could have made it three when he beat the offside trap after a Foran through ball and guided the ball inches wide.
Another goal did arrive seconds later when McKay thundered a shot from the edge of the area home in 78 minutes. Raven's cross was flicked on by Andrew Shinnie and the Northern Ireland international netted for his 20th of the season. Aberdeen showed little resistance as the hosts saw out the closing stages comfortably.
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