MGM had Mayfield listed at -250 as of last Monday, when the book took the odds off the board in compliance with Nevada Gaming Control regulations. The Westgate SuperBook closed Mayfield as even a bigger favorite at -2,000 on Sunday.
"That was because I wasn't looking to take any more money on him," Ed Salmons, assistant manager at the Westgate, told ESPN on Sunday.
USC quarterback Sam Darnold and Penn State running back Saquon Barkley were each favorites at different points of the season, but Mayfield finished strong, leading the Sooners to big victories over Oklahoma State and TCU.
Mayfield passed for 598 yards and five touchdowns against Oklahoma State two weeks ago, then backed it up with a 333-yard, three-touchdown performance against TCU on Nov. 11 to solidify himself as the favorite.
Salmons said he took two mid-four-figure bets on Mayfield when he was 7-2 and 5-2 earlier in the season and, overall, the book was looking at a loss if the Sooners quarterback hoists the trophy.
Nevada Gaming Control requires Nevada sportsbooks to halt betting on the winner of the Heisman Trophy on Nov. 19.
Mayfield made headlines Saturday when he grabbed his crotch and yelled obscenities at Kansas' sideline during the Sooners' 41-3 victory over the Jayhawks. He apologized after the game, calling his actions "unacceptable" and "disrespectful."
The Heisman Trophy ceremony will take place Dec. 9.