In an industry-wide gamble, Mackay was slashed from 40/1 to as short as 6/4 to be the next top-flight boss to leave his post.
The move came after it was reported the man credited with carrying out many of the club's recent singings, Iain Moody, had been suspended from his role as head of recruitment and replaced with 23-year-old Kazakhstani Alisher Apsalyamov, believed to be a friend of owner Vincent Tan's son.
Coral were left offering a best of 4/1 by 1200 BST.
Sky Bet's Dale Tempest, whose firm cut Mackay's odds to 2/1, said: "While Cardiff haven't had the greatest start this season, of more concern to Mackay has to be that the owner appears to be making decisions without the manager's agreement.
"When that happens, it's no surprise to see money across the board in the sack race."
Hill's spokesman Joe Crilly added: "This gamble has more than a whiff of speculation about it rather than anybody having any inside knowledge, but having said that, punters have been backing Malky all morning at 25/1, 6/1, 5/2 and 6/4."
Previous favourite, Fulham's Martin Jol, drifted out to a best of 5/1 (Sky Bet) as a result of the money for Mackay.
Cardiff are 5/2 (Sky Bet, BetVictor) to be relegated this season. They are currently 14th in the table, having beaten Manchester City already, but sit just one point above the drop zone.