Patrice Bernier and Marco Di Vaio scored first-half goals and Montreal held on for a 2-1 win over TFC on Saturday.
"We're happy we won but I think we only played one half today, so we'll have to give them credit," Bernier said.
"Toronto came out hungry in the second half but we made mistakes that gave them a chance to be in the game. I call them freebies. Troy made some saves but we made some easy mistakes in the final third."
The Impact became the first Major League Soccer team to win three games this season. Goalkeeper Troy Perkins has allowed two goals in three starts as Montreal has claimed three straight one-goal decisions.
The Impact, who opened with wins in Seattle and Portland to match their total road victories last year, remained perfect before a crowd of 37,896 at Olympic Stadium in their home opener.
"We won this game so we have this emotion after the game, but I saw the players and they're not happy," Montreal coach Marco Schalibaum said. "That's a good sign, I thought, because they know it wasn't good, the second half. But we won."
Robert Earnshaw scored on a penalty kick in the 68th minute to pull Toronto within one.
The Reds fell to 1-2, one week after Earnshaw scored twice in a 2-1 win over Sporting Kansas City at Rogers Centre.
"We certainly didn't play well in the first half," Toronto captain Darren O'Dea said. "It wasn't good enough. We put it right in the second half but we had a mountain to climb by then."
Bernier gave the Impact a 1-0 lead 34 minutes in after Toronto's Ashtone Morgan fouled Andres Romero in the penalty area.
Bernier, Montreal's MVP last season, hesitated on his penalty kick and sent Reds goalkeeper Joe Bendik diving to his right before putting the ball into an open right side.
"If you have a thing that's going well for you, then keep doing it," said Bernier, who converted each of his six penalty kicks last season in the Impact's MLS debut.
Di Vaio made it 2-0 just before the half. Montreal captain Davy Arnaud sent a ball forward for Di Vaio, who scooted around Bendik to score his first goal of the season in added time.
Much like Bernier, Earnshaw used a stutter step to get Troy Perkins in motion before shooting past the Montreal 'keeper's diving attempt. Earnshaw did a forward flip in celebration as he ran towards the Reds' cheering section in the end corner of the stadium.
"Both penalties were probably a wee bit soft, but they cancel each other out, and it was just a mental error right before half that made it a really big mountain to climb," Toronto coach Ryan Nelsen said. "To the credit of the guys in the second half, they worked ever so hard and I think they applied a lot of pressure and deserved the goal and obviously looked like the more likely team to score, unfortunately we left just too much of a mountain to climb."
Montreal's Felipe Martins had an opportunity to make it a two-goal lead in the 89th minute but he sailed his shot from the left side over the crossbar.
Bendik made a couple of big stops on Di Vaio earlier in the first, including one four minutes in and another in the 43rd minute.
Impact defender Alessandro Nesta left 11 minutes in because of a groin injury.
"I had a little problem with my adductor," Nesta said. "I think it's nothing special and I hope to play in the next game (in New York)."
He was replaced by Dennis Iapichino, who took over the left side of the back end as Hassoun Camara moved into the middle with Matteo Ferrari.
NOTES: The Impact wore their new third uniform, featuring a jersey with blue and black vertical stripes in honor of the team's 20th anniversary. Montreal, which made its MLS debut last season, honoured players from its previous championship seasons during the pre-game ceremony, including Matt Jordan, David Testo, Rocco Placentino, Eduardo Sebrango, Patrick Leduc, Antonio Ribeiro and John Limniatis.