Dylan Strome entered the zone 2-on-1 and fed Mantha, who scored from the far post at 7:34 to pull the Capitals within 3-1.
"We didn't get off to the start we wanted. We didn't just simplify, get pucks in," Washington defenseman Trevor van Riemsdyk said.
Sheary kept the puck on a 3-on-2 rush and beat Ullmark to the far side from the left circle to make it 3-2 at 12:43.
"I thought we did a good job kind of taking over the game there in that second half," Kuemper said. "We really dug in, which is a good sign after that early hole."
Bruins forward Jake DeBrusk left the game with an upper-body injury after a hit from Alex Ovechkin with 40 seconds remaining in the second period. Montgomery did not have an update after the game.
NOTES:Montgomery was 60-43-10 in 113 games (2018-20) with the Dallas Stars. … Pastrnak is the first player in Bruins history to have at least four points in two season openers (2-2-4 on Oct. 13, 2016). Boston's record for points in an opener is five (Phil Esposito, 3-2-5, Oct. 10, 1973; Brad Marchand, 2-3-5, Oct. 13, 2016). … Krejci has 517 assists to move past Wayne Cashman for sixth in Bruins history. … Bergeron's 401st goal is one behind Rick Middleton for third in Bruins history behind John Bucyk (545) and Esposito (459). … The Bruins are 9-2-0 in their past 11 season openers. … Defenseman Anton Stralman, who signed a one-year, $1 million contract with Boston on Wednesday, was with the team but did not play due to Visa issues. … Bruins forward Jakub Lauko played 11:36 and drew two penalties in his NHL debut. … Forward Pavel Zacha had an assist in his Boston debut. … Capitals forward Evgeny Kuznetsov played his 600th NHL game. … Kuemper played his 300th game. … Washington had won five straight season openers and earned points in eight in a row (6-0-2). … Strome played 19:54 and had an assist in his Capitals debut. … Defenseman Erik Gustafsson had an assist and played 14:50 in his Washington debut. … Forward Connor Brown played 19:28 and had two shots on goal in his Capitals debut. … Capitals defenseman Martin Fehervary had a game-high 10 hits and Ovechkin had nine. Washington outhit Boston 48-32. … Washington plays at the Toronto Maple Leafs on Thursday.