McMahon said he hadn't been back to Roy since graduating.
"It's nice to come back to a place I actually did graduate from, because I'm still missing that college thing," said McMahon, who is not in the BYU Hall of Fame because he was nine credits short of graduating.
Given the reception he got on a day dedicated to him, it hardly seemed like he ever left.
Students born a decade and a half after McMahon left Roy wore T-shirts bearing his likeness and sported headbands and sunglasses, McMahon's trademarks. The cheerleaders and dance team performed the "Super Bowl Shuffle," a song and dance made famous by the '85 Bears, who did indeed win the Super Bowl.
McMahon graduated from Roy in 1977 after leading the Royals to a region football championship as a senior and being named the Class 4A MVP in both football and baseball, as well as shining at basketball.
"He played quarterback like a linebacker," said Roy coach Fred Fernandes, who was a sophomore receiver for the Royals when McMahon was the senior quarterback.
Highlight tape flickered against a large white screen Friday, showing off McMahon's arm and strength.
McMahon may have been the greatest quarterback to ever come out of Utah, but he said Friday the position wasn't even his first choice.
"I always wanted to be a receiver," he said, "but I was too slow. A QB? It's a fun position. If the linemen block, it's fun. If they don't, it sucks. My youngest boy played two years of high school football he was a quarterback. But he was 6-5, 240 pounds, he was bigger than all of his linemen and he got his ass beat every game. I said, 'It's no fun when they don't block, is it?' "
McMahon suffered five concussions as an NFL player and is leading a group of former players who are suing the NFL over negligence related to head injuries.
"That's what I deal with every day, trying to get out of bed," McMahon said. "That's why I moved to Arizona the warm, dry heat feels so much better on my body. I'm having some symptoms now. I know I've got problems."
He said he struggles sometimes with memory.
The College Football Hall of Famer touched on some lighter topics, as well. While he was a three-sport star in high school he had a scholarship offer to play hoops at Utah State and played freshman baseball with the Cougars he didn't discover hockey until it was too late to start.
"That was a sport I'd have loved to play," McMahon said. "Hit a guy in the face with a stick and only get two minutes? I'd have lived in that penalty box."