The judges scored it 28-29, 29-28, 30-27 for Laprise, who finished strongly in what was a very close fight.
"I can't describe what I am feeling right now. People are asking me what it feels like to be the first winner of The Ultimate Fighter from Canada… and all I can say is I feel blessed," said Laprise. "Olivier was tough. I knew he would be, he's a southpaw and has a very difficult style to figure out. He is young and will come again.
"Right now, I feel so happy right now, so proud. I just had Dana White hand me a TUF finale trophy. I can't describe what's going on in my head."
Laprise (9-0) got on his knees in the middle of the cage after the fight. Aubin-Mercier (5-1), seemingly favouring his right foot, hung on to his cornermen.
The welterweight and middleweight finals of "The Ultimate Fighter Nations," which pitted Canada against Australian, were all-Canadian affairs after the Aussies were eliminated earlier.
Toronto's Elias (The Spartan) Theodorou stopped Edmonton's Sheldon Westcott via second-round TKO in the middleweight final.
After Theodorou (10-0) came into the music of The Red Hot Chili Peppers, Westcott (9-2-1) ran into the arena to Imagine Dragons, almost wiping out as he turned a corner. It was a frantic opening as Westcott looked to take the fight to the ground quickly, hoping to bring his submission game into play.
The bigger Theodorou fought him off and started punishing him with knees.
Theodorou's size and wrestling skills took their toll. He dumped Westcott midway through the second round and hammered him. The referee eventually stepped in to stop the beating at 4:41
Canadian coach Patrick (The Predator) Cote, who fought Australian coach Kyle Noke in a welterweight bout later on the Colisee Pepsi card, was runner-up to Travis Lutter on Season 4 of "The Ultimate Fighter," which featured veteran fighters looking for a second chance.
Aubin-Mercier stalked Laprise, who looked to counter from the outside, as he waited for the right moment to try a takedown. Laprise stuffed two attempts in the first round but was cut under the left eye as the crowd chanted his opponent's name.
Laprise was finally taken down a minute into the second round, to the delight of the crowd. But he quickly got back up. Aubin-Mercier kept coming but never got his ground game going.
Laprise denied another takedown to open the third but was bundled down soon after. Again he got back up quickly. Aubin-Mercier connected with kicks as Laprise continued to fight from the counter position.
Laprise finished the fight with a flourish of blows.
Wednesday's card marked the UFC's first visit to Quebec City.
In addition to being teammates on the show, Laprise and Aubin-Mercier have also trained together at Montreal's Tristar Gym, home to former UFC weltwerweight champion Georges St-Pierre.
"They're both my friends," said GSP. "Both very nice guys."
St-Pierre, however, picked Aubin-Mercier to win. The former champion has closer ties to Aubin-Mercier than Laprise — they have known each other longer, live close to each other and Aubin-Mercier was an important training partner ahead of St-Pierre's last fight against Johny Hendricks.
Aubin-Mercier, 25, does most of his training at Montreal's H2O Gym,
A former member of the Canadian judo team, he advanced to the TUF Nations final by beating a pair of Australians: Jake Matthews by decision and Richard Walsh by submission.
A native of Windsor, Ont., Laprise now makes his home in London, Ont., and trains mostly out of Adrenaline MMA Training and Fitness.
Laprise, 27, won a unanimous decision over Australian Chris Indich before defeating Kayan Johnson's jaw via a brutal knockout to advance to the final.
"It was difficult seeing the aftermath with Kajan and his jaw breaking, obviously," said Laprise.
But both knew what they had signed up for.
"That could easily just as well been me," Laprise said.
Johnson returns to the cage in June to face lightweight Tae Hyun Bang at UFC 174 in Vancouver.
Like others on the show, which wrapped up filming in December, Laprise was unable to talk about what happened until the episodes aired.
"Keeping it a secret for so long was definitely tough," he said.
Both Laprise and Aubin-Mercier have spent most of their fight career at lightweight (155 pounds) but competed at welterweight (170) on the show. Laprise normally walks around at 190 pounds.
"I'm probably one of the biggest lightweights in the division," said the five-foot-10 Laprise. "So me fighting at 170 is actually awesome."
Laprise added after the fight: "I'd like to drop down to 155 pounds now, I'm a little small for 170 pounds. But I will fight anyone the UFC want me to. I'm here to fight."
Fighting aside, Laprise is a man of faith. After weighing in Tuesday, he donned his trademark large cross to square off with Aubin-Mercier.
"I truly believe that God put me here for a reason. And that's to share his light through fighting. Everybody that sees me fight, they know I'm a Christian. Hopefully I represent God well."
He found religion at the age of 21.
"My life has changed dramatically since and I'm very grateful for that."
Living with 15 other fighters under the same roof with cameras everywhere for six weeks during filming of the TV show was a challenge however.
"It was tough, man," he said. "You're constantly on film, every single day."