The Sens took advantage of shaky goaltending from Canadiens netminder, Peter Budaj, who struggled Thursday at home in his first career NHL playoff start.
Bad goaltending was the theme for Montreal in this series. After stellar performances against Ottawa during the regular season, Carey Price was average in Games 1 to 4 and Budaj never looked comfortable in the Canadiens' net giving up six goals on 29 shots.
At the other end, Senators' goaltender Craig Anderson was solid again as the Canadiens came out buzzing around his net. He made several good saves early, including a glove stop off Rene Bourque, one of the Canadiens' best players in the series.
The series win was a first for many on the Senators, including Anderson and several other young players.
"It feels like a big relief. I knew it was eventually going to come you just didn't know when," said Anderson, 31, who stopped 33 of 34 Canadiens shots.
“It's a credit to our team in there and what kind of group of guys we have … Everybody pulling on the rope in the same direction."
It was an all-Canadian matchup filled with drama, even though it only lasted five games. For Senators head coach Paul MacLean, it was a difficult battle that means a lot to him.
“We just beat the Montreal Canadiens. To me that's huge. I gotta call my mom and tell her we beat her team," said MacLean, who won his first playoff series as an NHL head coach.
Sens silence noisy home crowd
The Senators gave Anderson some support early. Two minutes after the opening faceoff, Zack Smith tucked in a juicy rebound from a soft Matt Kassian shot to give the Senators the lead.
Ten minutes later, Cory Conacher poked a rebound into an open net to make it 2-0.
The pair of goals sucked the air out of the Bell Centre, even though the Canadiens were doubling the Senators in shots and were clearly the better team.
However, with just 15 seconds left in the opening period, P.K. Subban awoke the home crowd with a power-play goal to put the Habs on the board.
The Canadiens carried the momentum into the second period but Sens centre Kyle Turris quashed that with an ugly, and somewhat controversial short-handed goal.
After Tomas Plekanec pushed him into Budaj, Turris lay in the crease and Erik Condra shot the puck off his ribs and into the net. That was the third of the playoffs for Turris, who now has a three-game goal scoring streak.
Alfredsson iced the game in the third with a power-play goal to retake the team scoring lead with Erik Karlsson. Conacher added his second of the game and Condra the sixth and final goal, also on the power-play, in the final minutes.
The series win, the franchise's first since 2007, signalled a new era of Ottawa Senators success.
"We really changed the makeup of the whole team and (we're) going in a new direction. Last year was a great learning curve just to get into the playoffs for a lot of the guys," said Alfredsson, the Senators' 40-year-old captain.
“Now building on that … we're not going to be pretty every night but we seem to find a way and we always believe."
Battered Habs missing key players
Carey Price was missing Thursday after the Canadiens announced he would miss the rest of the series with a lower-body injury.
The injury to Price wasn't the only one facing the battered and bruised Canadiens.
Forwards Brandon Prust, captain Brian Gionta, Ryan White and Lars Eller also missed Game 5, as well as defenceman Alexei Emelin, who is out for the year.
Defenceman Eric Gryba, whose hit knocked Eller out for the series, played his first game at the Bell Centre since the hit in Game 1. But the fans didn't seem to notice when he was on the ice, so there were limited jeers.
The Senators franchise has now won seven straight times in Game 5 when leading a series 3-1.
They could still face as many as five teams in the next round — Pittsburgh, Washington, Boston, Toronto or the New York Rangers. Tickets for the Senators' second round home games go on sale Monday at 10 a.m. ET.