They were Pastrnak’s first two goals of the season and the first two on the man-advantage for Boston, which entered 0-for-10 through four games but finished 3-for-4 on Thursday.
Morgan Geekie added a goal and two assists, all on the power play, and Jeremy Swayman made 36 saves for the Bruins (3-2-0).
Jack McBain scored for the Mammoth (3-2-0), who entered with the NHL’s best power play (8-for-13, 61.5 percent) but went 0-for-7. Sebastian Cossa allowed three goals on eight shots before being relieved by Karel Vejmelka, who made 11 saves.
The Bruins’ sustained forecheck led to their first goal when Casey Mittelstadt made it 1-0 at 10:06 of the first period. His wrist shot from the left face-off dot beat Cossa to the short side.
Pastrnak’s power-play goal gave the Bruins a 2-0 lead at 14:22. His quick wrist shot from the top of the right circle beat a screened Cossa through the five-hole.
Mason Lohrei increased the margin to 3-0 at 15:51. He gained possession at the offensive blue line, broke down the right side and cut toward the net at the bottom of the circle before scoring with a quick wrist shot to the stick side.
Vejmelka replaced Cossa following the goal.
Pastrnak’s second power-play goal made it 4-0 at 4:40 of the second period. His one-timer off a Geekie cross-ice saucer pass beat Vejmelka high glove.
Geekie scored on the man-advantage for a 5-0 lead at 7:57, and Matthew Poitras pushed it to 6-0 at 14:08 off a Pastrnak pass.
McBain scored at 9:48 of the third period for his first point of the season, making it a 6-1 final.
