Mitchell only had hit 2 of his 10 attempts from the field when Corner Canyon (17-7) started its 11-2 run that ended with Mitchell's trey and a 66-65 lead with 1:54 left. But Mitchell scored six of those 11 points.
"More than a little frustrating," said Maple Mountain coach Johnny Averett about the momentum shift midway through the fourth quarter. "This is the kind of loss that'll really haunt you forever because you know you had it. All you had to do was hold the ball because they're gonna foul you.
"We coughed it up again and again and again. And forced some bad shots, which gave them opportunities."
Maple Mountain (17-7) had a chance to regain the lead, but Tyler Boyack missed a shot with 23 seconds left.
Corner Canyon's Ammon Jensen sunk a pair of free throws for a 3-point lead with 11 seconds left then teammate Michael Scheffner hit 1 of 2 from the line with 6 ticks remaining after he stepped in front of a Golden Eagles pass to thwart Maple Mountain's chance to tie.
The Eagles got a putback score at the buzzer, but the disappointment already had begun to set in.
Boyack led Maple Mountain with 16 points, while Dawson Beutler hauled in 18 rebounds for the Eagles.
Jensen tallied 16 points, while Zachary Wilson scored 14 points in the win.
"We challenged them to believe, and they stepped up in the end to make plays," Chargerscoach Dan Lunt said after leading his team to the first semifinal berth in school history. "We got some steals and turnovers, which led to some scores because we were pretty stagnant offensively. We struggled offensively. We just found a way to get it done when we needed to, thank goodness."