The victory before 13,698 fans crowded into the PEPS stadium also stopped Laval's 70-game winning streak at home. Laval had won the Canadian Interuniversity Sport football title — the Vanier Cup — the last two years.
The game, in which the only scoring came from field goals, was won in the trenches in a hard fought, defensive battle.
"We had to find a way to control the line of scrimmage," coach Danny Maciocia said after the Carabins' first conference title. "The Rouge et Or won all those championships because of their play on the offensive and defensive lines.
"That's not to say they don't have impact players in other positions, but it starts on the lines."
Regulation time ended at 9-9 after Laval's Louis-Philippe Simoneau's 21-yard field goal with 1:01 left to play following a fumble recovery.
Overtime saw each team get a chance to score starting from the 35 yard line. The Carabins got their field goal on their turn with the ball, but Laval's Boris Bede left a 47-yard attempt short. It was Montreal's first win in six playoff meetings with the Rouge et Or.
"All good things come to an end," said Laval coach Glen Constantin. "We were up against a very determined team.
"Two good defences. I'm proud of our guys. There were no touchdowns and field position and the wind were factors. But I tip my hat to Montreal. They played a very good football game. We will be back."
Next Saturday, the Carabins will play in the Uteck Bowl against the Manitoba Bisons, who beat the Calgary Dinos 27-15 for the Hardy Cup.
The Carabins led 6-0 at the intermission.
They scored on their first possession on Simoneau's 35-yards boot and added a 22-yarder in the second quarter. he also missed a 36 yard attempt against the stiff wind.
Carabins quarterback Gabriel Cousineau completed 25 of 38 passes for 312 yards. Laval's rookie pivot, Hec Crighton candidate Hugo Richard, went 17- for 32 for 264 yards.
The Vanier Cup is set for Nov. 29 at Percival Molson Stadium in Montreal.