Former Canadien Smith-Pelly scored the winner with 5:54 remaining on the clock when he deflected Connor Hobbs’ point shot over Charlie Lindgren, who lost sight of the puck.
Tom Wilson added a fourth for Washington into an empty net.
Carey Price, in his first game of the pre-season, allowed two goals on 10 shots before being replaced by Lindgren at 9:22 of the second period. Lindgren made 10 saves in relief.
Philipp Grubauer stopped 20 of 21 shots. His replacement Pheonix Copley made 21 saves.
Montreal outshot Washington 43-22.
Special teams were the difference on Wednesday as the first four goals of the game were scored on the power play.
Kuznetsov got the visitors on the board at 5:07 of the first as he danced around Habs defenceman Mark Streit to move clear on goal.
Vrana made it 2-0 when his centering pass deflected off Brandon Davidson’s stick into the roof of the net.
Hudon got one back for the Canadiens at 1:16 of the second on a one-timer in the slot off a perfect feed from defenceman Victor Mete.
Ales Hemsky found Pacioretty all alone next to Copley’s goal with a precise saucer pass at 11:37 of the middle frame for the easy equalizer.
The Capitals took nine minor penalties in the game to Montreal’s four.
It was the first official game in a Canadiens uniform for 22-year-old forward Jonathan Drouin, who was traded for defensive prospect Mikhail Sergachev in the off-season.
Andrew Shaw (neck) and Noah Juulsen (foot) were not in the lineup for the Canadiens. Both players were injured in Montreal’s first pre-season game on Monday.