The last Mannion dagger was a six-yard touchdown pass to Brandin Cooks in overtime to win the game.
Oregon State's players rushed onto the field while Utah's players stood stunned in disbelief.
It was a painful loss for a Utah team that rallied time and time again in the game.
Trailing by 17 points at one juncture, the Utes battled back and forced overtime when Travis Wilson scored on a 9-yard touchdown run with 21 seconds remaining.
It was just one of the many highlights Utah's sophomore quarterback delivered, not all of which were good. Wilson was trying to win the game for his former childhood teammate Nick Pasquale, a UCLA receiver who was fatally struck by a car a week ago.
His friend would have been proud of his efforts.
Wilson threw three interceptions in the second half, two of which led to Oregon State scores, but he was still effective enough with his arm and his feet to keep Utah in the game.
He finished the night 19 for 33 for 279 yards and two touchdowns passing and also led the Utes with 13 rushes for 142 yards and three touchdowns.
But he and the Utes just couldn't overcome Mannion, a junior who was 27 for 44 for 443 yards and five touchdowns.
Utah coach Kyle Whittingham credited Mannion for his play but said the turnovers were the difference in the game noting the defense needed to create takeaways.
"The bottom line is we didn't get it done," he said. "It was a tough one to swallow but we have to regroup and come back Monday with resolve and go to work."
The Beavers had their own emotional moments in the game too as running back Storm Woods suffered a head injury and was removed from the field by ambulance. He did have movement in his extremities according to school officials.
Woods finished the game with 12 carries for 34 yards as Mannion relied on the passing game for the majority of his damage.
Mannion and receiver Brandin Cooks hooked up for two touchdowns in regulation. Cooks, fourth in the nation averaging 144 receiving yards a game, finished the night with nine catches for 210 yards and three touchdowns as he found ways to get open every time the Beavers needed a big play.
One was a 48-yard pass play from Mannion on fourth-and-1 that led to a 20-yard field goal that put the Beavers ahead 37-31 with 10 minutes remaining.
After the Utes responded with a seven-play, 85-yard drive that ended with a 24-yard touchdown run by Wilson that put the Utes ahead 38-37, Mannion came right back with some big plays of his own in the final minutes.
The key play was a 13-yard pass to Richard Mullaney on fourth-and-9 with less than four minutes remaining.
The Beavers were rewarded for the gutsy call when Mannion connected with Cooks on an 18-yard touchdown play, going up 45-38 with 2:29 on the clock.
That point could easily have marked the point where the Utes folded.
But Wilson kept the Utes in the game. He threw a 29-yard pass to Dres Anderson, then running back James Poole rushed 18 yards to Oregon State's 8-yard line.
Two plays later, Wilson found the right corner of the end zone on a 9-yard run with 21 seconds remaining that tied the game at 45-45.
Oregon State failed to score in the final seconds, sending the game into overtime.
All the momentum and emotion seemed to be on the Utes' side, but they had to settle for a 41-yard field goal from Andy Phillips in overtime.
That was the only chance Mannion needed as the quarterback ultimately found his man, and left Utah still searching for that elusive Pac-12 opening win.
Whittingham said he was proud of the way Wilson and his teammates fought in the game and believes their future looks bright. But at least on Saturday, all the Utes could think about was the painful loss.
"They are hurting," Whittingham said. "They spilled their guts to the bitter end."