Pastrnak added an assist, Hampus Lindholm had a goal and an assist, and Charlie McAvoy had two assists for the Bruins (11-7-0). Jeremy Swayman made 30 saves.
Auston Matthews left midway through the second period with a lower-body injury for the Maple Leafs (8-8-1), who lost their third straight. Anthony Stolarz allowed three goals on 10 shots before leaving after the first period with an upper-body injury and being replaced by Dennis Hildeby (19 saves).
Pastrnak scored the milestone goal at 49 seconds of the second to give Bruins a 4-1 lead. He was sent in by a one-touch pass from Morgan Geekie at center ice and beat Hildeby on a breakaway. As Geekie collected the keepsake from the Toronto net, the Boston players left the bench to congratulate Pastrnak.
Pavel Zacha’s power-play goal gave the Bruins a 1-0 lead at 4:27 of the first. Pastrnak’s pass from the right face-off circle deflected off Zacha’s skate and got past Stolarz.
Steven Lorentz tied it 1-1 with a short-handed goal at 6:41. He stripped McAvoy of the puck and beat Swayman to the stick side with a wrist shot from the high slot.
Lindholm put Boston back in front 2-1 on the man-advantage at 9:32. His wrist shot from the top of the left circle beat Stolarz to the top right corner.
Alex Steeves scored his first of the season for a 3-1 lead at 17:23. His wrist shot from the left circle got through Stolarz’s five-hole.
After Pastrnak extended the lead to three, Toronto cut it to 4-2 on Bobby McMann’s power-play goal at 18:58.
Oliver Ekman-Larsson then pulled the Maple Leafs within 4-3 at 3:32 of the third period with a wrist shot from the point before Pastrnak’s power-play goal at 9:48 made it a 5-3 final. He scored with a wrist shot from low in the left circle off a cross-ice pass by Mark Kastelic.