The 6-foot-6, 225-pound Mattingly told the Irish staff, then called Mead High coach Sean Carty to share the news.
"He's pretty excited," Carty said. "I think growing up as a Catholic kid, it was a dream of his and he finally went and visited there and figured that was the place to be."
"I think he was open to all the other offers and expressed to me and other folks that were after him that he wanted to go and visit and meet the Alabama staff and the Oklahoma staff," Carty said. "He met the Oregon staff. He was very excited about all those prospects, and when he got to Notre Dame he was just enamored with the whole deal.
"He told me that he was going to cancel his trip to Oklahoma. He said he was going to call the rest of the schools and say he made a decision and that was it."
Mattingly will play on the edge of Notre Dame's 3-4 defense, mostly as an outside linebacker. He made 62 tackles and intercepted four passes for Mead as a linebacker, and doubled as a tight end and finished with 10 catches for 245 yards.
"He's an extremely aggressive player on defense," Carty said. "He'll chew threw barbed-wire to get to the quarterback. He's a technician in drills. He likes to do things right. He's very disappointed in himself when he misses, or doesn't do something right technically and will work really hard to correct it. Those are his best assets. He's a perfectionist when it comes to that kind of stuff."
Carty experimented with playing Mattingly in the middle of his defense, but said he best suited to play outside linebacker, with a move to defensive end possible.
"I really feel that he's better in space than tied up in the middle," Carty said. "Some of the schools talked about putting him in a three-point stance, and Notre Dame talked about that as well. I think that will be ok if he can add some weight, but he's really, really good in space."
When it came to the technical aspect, Carty had high marks for Mattingly.
However, overall development and the ability to pick up a college scheme will be the biggest challenge early in Mattingly's career.
"Defensively, he's pretty solid," Carty said. "I think our scheme we use is very simple. I think that getting a grasp of everything he'll need to learn at the college level …I don't think it's any different from any other kid, but that will be a challenge for him. His eyes will open up.
"The game speed changes, the athleticism changes. He'll grow and develop and all that. He's a good tackler. He pays attention. He's very coachable. He doesn't think he knows it all."