Armstrong was drafted by the Pittsburgh Penguins in the first round of the 2001 NHL Entry Draft as the 21st pick overall. In the 2005–06 season, Armstrong made his NHL debut, and had a superb rookie season, in which he played 47 games and tallied 40 points and had a team-high plus-minus rating of +15. Armstrong had three overtime goals for the Penguins in the 2006–07 season, beating Kari Lehtonen of the Atlanta Thrashers, Ed Belfour of the Florida Panthers and Henrik Lundqvist of the New York Rangers. The Penguins and Armstrong avoided an arbitration hearing in the 2006–07 off-season by re-signing him to two-year, $2.2 million contract. Armstrong became very close friends with former teammate Sidney Crosby while playing for the Penguins. On January 1, 2008, Armstrong scored the first ever outdoor Winter Classic goal in the inaugural event. Pittsburgh traded Armstrong on February 26, 2008, to the Atlanta Thrashers, alongside Angelo Esposito, Erik Christensen and a first-round draft pick, in exchange for Marián Hossa and Pascal Dupuis. He then scored 11 points in 18 games to finish out the 2007–08 season with Atlanta. On July 16, 2009, Armstrong re-signed with the Thrashers to a one-year, $2.4 million contract. He served as one of the team's alternate captains during the 2009–10 season. On July 1, 2010, Armstrong signed as an unrestricted free agent with the Toronto Maple Leafs on a three-year, $9 million contract. With his first two seasons largely affected by injury and inconsistent play, on June 30, 2012, he was bought-out by the Maple Leafs from his final season of his contract. With the opening of free agency the next day, July 1, 2012, Armstrong signed a one-year, $1 million contract with the Montreal Canadiens. During the lock-out-shortened 2012–13 season, Armstrong played mostly on the fourth line. Although he contributed only 5 points in 37 games, Montreal made the 2013 playoffs. On July 27, 2013, Armstrong left the NHL as a free agent and signed his first and only European contract with the Växjö Lakers of the Swedish Hockey League.
International play
Armstrong was a member of Canada's 2007 IIHF World Championship-winning team. His only goal of the tournament was the game-winning goal in the gold medal game against Finland in Moscow, which Canada won 4–2. He scored the first ever outdoor Winter Classic goal in the inaugural 2008 event.
On June 23, 2001, selected by the Pittsburgh Penguins in the first-round of the 2001 NHL draft.
On August 12, 2005, re-signed by the Pittsburgh Penguins.
On July 21, 2006, re-signed by the Pittsburgh Penguins to a 1-year contract.
On July 12, 2007, re-signed by the Pittsburgh Penguins to a 2-year/$2.4 million contract.
On February 26, 2008, traded by the Pittsburgh Penguins with Angelo Esposito, Erik Christensen and a 2008 first-round pick to the Atlanta Thrashers in exchange for Marián Hossa and Pascal Dupuis.
On July 16, 2009, re-signed by the Atlanta Thrashers.
On July 1, 2010, signed with the Toronto Maple Leafs as an unrestricted free agent to a 3-year/$9 million contract.
On June 30, 2012, the Toronto Maple Leafs bought out his contract, making him an unrestricted free agent.
On July 1, 2012, signed by the Montreal Canadiens as an unrestricted free agent to a 1-year/$1 million contract.
