Andrew Hammond stopped 26 shots to for Montreal to improve to 2-0-0 in two starts this season. The Canadiens' win streak follows a 10-game losing streak (0-8-2) that saw Dominique Duharme fired as coach and replaced by Martin St. Louis.
''I feel like we've gotten better and better and better and we're doing it consistently now,'' St. Louis said. ''We come to work, take care of the team, play the game the right way. You're going to be in a lot of games.''
Colin White scored for Ottawa in his season debut after recovering from a preseason shoulder surgery, and Matt Murray had 30 saves. The Senators have lost six of their last nine (3-5-1).
''We didn't have the puck in their zone enough to give our (defensemen) a breather,'' Senators coach D.J. Smith said. ''We had bad pinches and some (players) not covering guys. ... There were a lot of things that were good, but offensively we just weren't very good.''
Hammond, who spent three-plus seasons in Ottawa from 2013-17, had nine saves in the first period, 10 in the second and seven in the third.
''It's natural, any time you're playing your former team you'd like to win,'' Hammond said. ''Obviously very happy that we were able to do that tonight. Just a really grinding game by us.''
Lehkonen got the Canadiens on the scoreboard first for the fifth straight game as he got the rebound of Ben Chiarot's shot from the blue line and tucked it past Murray with 3:13 left in the first period.
White tied it with a power-play goal at 1:23 of the second as he got a pass from Connor Brown in the slot and quickly fired it past Hammond.
Lehkonen regained the lead for Montreal as he beat Murray on the blocker side with a slap shot from the high slot with 5:39 remaining in the middle period.
Canadiens: At Winnipeg in the second of a five-game trip.
Senators: At Tampa Bay to open a five-game trip.
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