McCormack's first score came on the Garnets' opening drive, giving them a 7-0 lead they'd nurse into the half.
Jack O'Callaghan returned the second-half opening kickoff 80 yards, catching it just in front of the goal line and taking it to Harrison's 21. Three players later, McCormack scored on an 8-yard run.
"(The kickoff return) was very big," Garr said. "Normally, their guy kicks it in the end zone. We were hoping he'd be a little short on one. It changed the game."
McCormack's 4-yard touchdown run on Rye's next possession put the game out of reach as a 21-0 deficit was far too much for the Huskies (4-2) to overcome.
Harrison scored in the fourth when Tanner Knox ran in a 2-yard touchdown. On the ensuing kickoff, Jack Molloy's arching onside attempt was snared by Harrison's Dennis Rinaldi. But Rye took possession because, according to high school rules, an onside kick must hit the ground before the kicking team can recover it.
The Garnets would convert four first downs and run out the final 8:25 left on the clock.
It was the fourth straight win for Rye, which hadn't lost to a Section 1 opponent since 2002 until its Week 3 loss at Eastchester.
"We really grew from that game," Rollins said. "Eastchester set us straight. We're not the same team anymore. We've come back from that with a different attitude. We want to prove to everyone that we're still a good team."
The win provides plenty of momentum for the Garnets, who moved up to Class A after winning the last six sectional titles in B.
"Everyone here realizes what it means to beat Harrison," McCormack said. "So the Section 1 streak is done, but the Harrison streak isn't. This is a huge deal for us. It's a community win. Nobody can take it away from my class or away from the previous classes. Winning this is just awesome."