The high-scoring guard from Sarasota, Fla., Cardinal Mooney High School gave U of L head coach Rick Pitino a commitment on Friday morning.
"Proud Cardinal day," McMahon's father, David, told Cardinal Authority.
The 6-foot-1, 175-pound McMahon has created quite a buzz this season in Florida, averaging just over 30 points a game at Cardinal Mooney.
Pitino found McMahon recently - after a tip from ESPN's Dick Vitale - when he watched in him a game at Cardinal Mooney. Assistant coach Mike Balado had seen him in practice and assistant coach Wyking Jones and Balado saw McMahon on Thursday night.
"It's all happened so fast," McMahon said on Thursday night.
And that was before his offer from Pitino, who called about an hour after Cardinal Mooney's overtime win to put them in the Class 3-A quarterfinals.
"Louisville was great when I went up there," said McMahon, who made an official visit on Feb. 1. "I just loved everything about it."
McMahon, who only weighs 175 pounds but has a vertical jump of 39 inches, averages just under 30 points a game for a 23-3 Cardinal Mooney team that plays in Class 3-A in Florida. He scored 26 against LSU signee - and former U of L commit - Antonio Blakeney earlier this season in a game where he fouled out in the third quarter.
An unranked prospect, McMahon said Pitino had a comparison that made him smile.
"He said I reminded him of Mark Price when he was in high school," McMahon said. "That's a very big compliment. At first, he said someone told him I was like Billy Donovan but when he watched me he said, 'That's not Billy Donovan, that's a young Mark Price'."
U of L now has four new players coming in this summer, including guard Donovan Mitchell and forwards Ray Spalding and Deng Adel.
This story will be updated.