Liam struck four minutes into the second half.
The victory over Livingston gave Hearts a five-point cushion in third place.
Craig Gordon produced a number of great stops to prevent the hosts from going ahead, but Liam Boyce's well-taken second-half finish decided the game.
McKay was given a second chance by the ref as his side recovered from a bad first half.
They had a winless run of six games.
Ross County has played a game more than David Martindale's men.
Reaction as it happened.
The first half belonged to Livingston, they were organised, intelligent and lively, and they were the ones who could not match the intensity of the other team.
Stephane Omeonga is a diminutive specimen, but he patrolled the middle like a colossus. It felt like there were three of him in that first 45 minutes.
Gordon comes to their aid when they need him. Scotland's goalkeeper has been praised for his renaissance, but his three-save here might be the best of his career.
Gordon blocked the shot after Andy Shinnie raced from his line.
Craig grabbed the rebound and slammed it off a post. Livingston regathered, Holt had space 25 yards out and his rasping, top corner-bound effort was tipped on to the same upright by the flying Gordon.
Had John Souttar not slid in front of him to knock the ball out of play, Alan Forrest's effort from the left would have been pawed away by the Hearts goalkeeper.
Those inside the Tony Macaroni Arena were given 30 seconds of sheer roller-coaster fun. Gordon leaped to his feet. The 4,000-strong travelling support roared back, "Scotland's number one."
After a flurry of near misses, he decided his midfield had been bullied enough. Peter Haring was sent in place of Ben Woodburn.
By the half-time, David Martindale had watched his side take 11 shots. They could not convert their dominance into a lead and they were not able to break the door that Gordon kept them out of.
They were punished four minutes into the new half. Souttar had time to step into the middle of the field and hit the ball to McKay. The winger tried to get in the way of Boyce. A shot at a target. Ruthless.
The goal gave Hearts new-found belief and the momentum swung dramatically.
Haring's influence was telling as he harryed the Livingston midfield to allow McEneff to force Stryjek into a save. McKay did lace beyond the Livingston goalkeeper but the goal was not allowed.
Forrest raced away on the counter, but after cutting inside, he was faced with a fine Gordon save, low to his left. Soon after, Montano blasted narrowly wide.
By that time, hearts were a different beast. The meekness of the first half had changed. Souttar, Haring, and McKay had a lot to do with it.
McEneff could have settled the issue when he hit over on the break and when his drive was tipped clear by Stryjek.
McKay might have killed it after being neatly set up on the 18-yard line by Alex Cochrane, but he should have done better when he rounded Stryjek, but only clipped into the side net.
The final quarter was relatively unflustered as the early storm had weathered. A victory hard-won and rowdyly celebrated that propels them five points clear of Motherwell.
Craig Gordon was the man of the match.
Gordon kept his team level with his magnificence, and dealt with any Livingston counter-punches as his team-mates began to dominate.
What did we learn?
Three games and three losses into the season, Martindale proclaimed his team would not be demoted. Their first-half performance was exceptional, clever tactics brilliantly executed, but their inability to strike proved fatal.
Livingston's spirit has not been splintered by a bad run of form, but more power is needed.
The way his team were humiliated in the beginning will have spooked him, he was given a yellow card for his frustration.
He had to change things. The introduction of Haring gave his players more bite in the middle of the game. Winning in difficult circumstances shows the depth of the quest for third.
What's next?
Livingston will host Hibs on Wednesday and then travel to face Dundee United on Saturday after the Leith side was hit by a Covid-19 outbreak.
The Rangers and Hearts game will be played the following day.
Livingston
There is a Hearts of Midlothian.
Line-ups.
Match statistics.
Live text.
Formation 4-4-2.
2Devlin5Fitzwater4Parkes
21McMillan18Holt33Omeonga
32Stryjek
2Devlin.
5 Fitzwater.
4 parks
29Penrice
Substituted forMontao at 63' minutes.
The book was written at 32 minutes.
Substituted for Panayiotouat.
17 forrest
Substituted for Anderson at 63 minutes and 73 minutes.
Shinnie.
Substitutes.
6Obileye.
9 Anderson.
11Montao
Williamson
15Panayiotou
24 Kelly.
36 Maley.
Formation 3-4-3.
4Souttar19Halkett3Kingsley
2 Smith8McEneff14
Gordon.
The book was written at 78 minutes.
19Halkett
3Kingsley.
Substituted forMoore at 17 minutes.
McEneff was 8.
The book was written at 90 minutes.
17Cochrane
18McKay
Substituted for Gnanduilletat for 80 minutes.
Substituted forHaring at 29 minutes.
Substitutes.
5 yelling.
7Walker
11Mackay-Steven.
13Stewart.
15Moore.
21Gnanduillet.
30Ginnelly.
Livingston and Heart of Midlothian played a match.
The second half ended with Livingston 0 and Heart of Midlothian 1.
The yellow card for a bad foul is shown to the man.
The person is foul by the person.
Andrew Shinnie won a free kick.
An attempt was missed. The shot from the difficult angle on the right was close, but missed to the right. Following a fast break, assisted by a man.
An attempt was missed. The shot from the left side of the box is close, but misses to the left. The assistance was given by Barrie McKay.
It was Livingston who was trying. Bruce Anderson is caught off guard by a through ball.
Stephen Kingsley won a free kick.
Foul by Nicky Devlin.
The yellow card for a bad foul was shown to Armand Gnanduillet.
Foul by Armand Gnanduillet.
The defensive half is where the free kick was won by Montao.
Foul by Taylor Moore.
Andrew Shinnie won a free kick.
The hand ball was thrown by Nicky Devlin.
The Corner is from the Heart of Midlothian. Max Stryjek Conceded.
The attempt was saved. The shot from outside the box is saved in the bottom left corner. They were assisted by Armand Gnanduillet.
An attempt was missed. Bruce Anderson's shot from the left side of the box is wide to the right. The assistance was given by Harry Panayiotou.
The Heart of Midlothian. Liam Boyce is replaced by Armand Gnanduillet.
