Marchand took a sharp-angled pass from David Pastrnak and flicked it past out-of-position Maple Leafs goalie Frederik Andersen from the left circle. Jake DeBrusk scored early in the third to seal the Bruins' win.
"We've played together for a while now, and (I) kind of know when he's thinking shot or pass," Marchand told NESN about his two-on-one play with Pastrnak. "So I just tried to find a good lane. He made an incredible pass."
Boston can close out the best-of-seven series at home in Game 5 on Saturday night.
Marchand and DeBrusk both increased their postseason goal tally to two. Torey Krug added his first playoff goal for the Bruins, who haven't advanced to round two of the Stanley Cup playoffs since the 2013-14 season.
Pastrnak had two assists to give him a playoff-leading 11 points.
Tomas Plekanec scored his first goal of the postseason for the Maple Leafs, who hope to avoid their second straight first-round exit. Toronto was eliminated by Washington in six games last season, its first playoff appearance since a first-round loss to Boston in 2012-13.
Bruins goalie Tuukka Rask finished with 31 saves. Andersen had 18.
Boston played without Selke Trophy finalist Patrice Bergeron, who was a late scratch due to an upper-body injury. The four-time winner of the award given to the league's best defensive forward was the Bruins' third-leading scorer this season with 30 goals and 33 assists.
"When Bergeron didn't play tonight, you're set up pretty good," Maple Leafs coach Mike Babcock said. "You've got to find a way to win. This is in our building, you've got to win."
Toronto's Leo Komarov (lower body) missed his second consecutive game.
Seconds after the Bruins' goal song "Kernkraft 400" played on the Air Canada Centre speakers during a stoppage in play, Marchand put Boston up 2-1 with 3:05 left in the second.
DeBrusk scored 4:17 into the final period.
Krug struck quickly with Boston's first goal on a wrister from the top of the left circle just 28 seconds into the game. Plekanec answered for Toronto on a one-timer from the slot with 12:17 left in the first.
--Field Level Media
Boston | Toronto | |
David Pastrnak 2 | Points | Tomas Plekanec 1 |
Jake DeBrusk 1 | Goals | Tomas Plekanec 1 |
David Pastrnak 2 | Assists | Patrick Marleau 1 |
N/A | Power Play Goals | N/A |
N/A | Short Handed Goals | N/A |
Tuukka Rask .969 | Save Percentage | Frederik Andersen .857 |
Tuukka Rask 31 | Saves | Frederik Andersen 18 |
Team | Shots | Goals | Power Play | Penalty Kill | Penalty Mins | Face Offs Won |
Boston | 21 | 3 | 0-0 | 1-1 | 2 | 24 |
Toronto | 32 | 1 | 0-1 | 0-0 | 0 | 41 |