The form book would have had Livi as the heavy favourites going into the contest, having won their last five matches in all competitions, but the Staggies got their just rewards for improved performances in recent weeks, ending an eight-match winless streak.
While Livi remained unchanged from their 1-0 win over Motherwell last weekend, County made six alterations and changed their shape to a back five to try and stop a side they had already conceded eight goals to in the league this season.
But the Staggies did not have to wait long to make an impact at the other end, as they went ahead after just three minutes.
Sean Kelly delivered a cross from the left into a busy penalty area, but captain Marcus Fraser had time to take the ball down with a lovely touch and lay it off for McKay to finish.
That goal gave County something to hold on to early on, and they might have been thankful that cushion was there because there was not much quality to the play in the first half.
But as half-time approached, Livingston started to come into the game more, getting the ball down and passing it around.
Lyndon Dykes, so often impressive for the Lions this season, began to make an impact, but he could only find the side-netting from close range after a mazy run.
And it was the hosts who found the all-important second goal on the hour mark.
Iain Vigurs won the ball back as Livingston were trying to push up the park, and played in McKay who had broken the offside trap.
Livi's defenders seemed to stop expecting the flag, but it stayed down and McKay went round the keeper to find the net.
Both sides defended fairly well from there, with only Dykes being left open and heading towards goal for what was an easy save in the end for Nathan Baxter.
That was representative of the evening as a whole for Livingston, who barely threatened County's goal.
Frustration started to show towards the end as they picked up multiple bookings, but they can have no complaints about the end result as they left Dingwall empty-handed.