Gagné was drafted 22nd overall by the Philadelphia Flyers in the 1998 NHL Entry Draft and made the Flyers out of training camp prior to the 1999–2000 season.
His rookie season saw him play primarily at centre and score 20 goals and 28 assists in 80 games, good enough to be named to the NHL All-Rookie Team.
He contributed 5 goals and 5 assists in 17 games during the playoffs before the Flyers lost in seven games to the New Jersey Devils in the Eastern Conference Finals.

The following season, Gagné was converted from centre to wing.
Playing on a line with Mark Recchi and Keith Primeau in the 2000–01 season, Gagné scored 27 goals and 32 assists in 69 games and took part in his first NHL All-Star Game.
During the All-Star Game, he scored two goals including the game-winning goal for the North American team.
2001–02 saw him score 33 goals and tally a career high 33 assists in 79 games.
A nagging groin injury limited Gagné to 46 games and career lows in goals, assists and points in 2002–03.
The next season, Gagné recorded 24 goals and 21 assists in 80 games.
He also scored an important overtime goal in Game 6 of the Eastern Conference Finals against the Tampa Bay Lightning.
However, the Flyers were eliminated in Game 7.

Once the 2004–05 NHL lockout came to an end, Gagné assumed a greater role on the Flyers and played most of the year as the left winger on the top line with Peter Forsberg and Mike Knuble.
The line became known as the "Deuces Wild" line due to their numbers (12 [Gagné], 21 [Forsberg] and 22 [Knuble]) and Gagné responded with his best season as a pro with 47 goals and 79 points in 72 games.
He was awarded the Bobby Clarke Trophy, which is awarded to the Flyers Team MVP.
In the off-season, Gagné signed a five-year contract extension worth over $25 million.
Despite not having Forsberg as his centre for much of the 2006–07 season due to injury and Forsberg's eventual trade to the Nashville Predators, Gagné scored 41 goals and 27 assists in 76 games and won his second consecutive Bobby Clarke Trophy.
He also played in his second NHL All-Star Game.

In the 2007–08 season, Gagné recorded 18 points in 25 games before being shut down for the year on February 20 after being told to sit out by doctors.
Although it was believed Gagné had suffered three concussions in five months, he was told by doctors that he only had one and further injuries re-aggravated the symptoms.
He suffered the initial concussion on October 25, 2007, when he was hit in the jaw with a check from Florida Panthers defenceman Jay Bouwmeester.
Gagné sat out four games and returned only to be hurt again on November 7.
After missing the next 26 games, he was re-injured on his first shift back on February 10, 2008, against the Pittsburgh Penguins.
 Gagné follows after several previous Flyers player who have suffered serious concussions.
Former captain Keith Primeau had his career ended prematurely due to concussions suffered while playing in Philadelphia, while Jeremy Roenick nearly retired after suffering one himself.

Gagné had a strong return in the 2008–09 season, scoring 34 goals, 74 points and posting a +21 in 79 games.
However, injury troubles persisted in 2009–10.
 Less than a month into the season, he was placed on the injured reserve after being diagnosed with a double hernia in his groin in late-October 2009.
Although the Flyers' orthopedic surgeon recommended surgery, Gagné opted for a second opinion with his personal doctor in Montreal.
Soon after, Gagné opted for surgery, which was performed on November 3.
The surgery repaired the two small hernias in his right groin laparoscopically through his belly button, while reinforcing his rectus abdominis on both sides.
Gagné returned to action on December 19 against the New York Rangers after missing 24 games.
He scored his third career hat trick and first career natural hat-trick just 11 days after returning from injury against the Rangers, December 30, 2009.
He finished the season with 40 points in 58 games.
After defeating the second-seeded New Jersey Devils in the opening round of the 2010 playoff
Gagné was drafted 22nd overall by the Philadelphia Flyers in the 1998 NHL Entry Draft and made the Flyers out of training camp prior to the 1999–2000 season. His rookie season saw him play primarily at centre and score 20 goals and 28 assists in 80 games, good enough to be named to the NHL All-Rookie Team. He contributed 5 goals and 5 assists in 17 games during the playoffs before the Flyers lost in seven games to the New Jersey Devils in the Eastern Conference Finals.
The following season, Gagné was converted from centre to wing. Playing on a line with Mark Recchi and Keith Primeau in the 2000–01 season, Gagné scored 27 goals and 32 assists in 69 games and took part in his first NHL All-Star Game. During the All-Star Game, he scored two goals including the game-winning goal for the North American team. 2001–02 saw him score 33 goals and tally a career high 33 assists in 79 games. A nagging groin injury limited Gagné to 46 games and career lows in goals, assists and points in 2002–03. The next season, Gagné recorded 24 goals and 21 assists in 80 games. He also scored an important overtime goal in Game 6 of the Eastern Conference Finals against the Tampa Bay Lightning. However, the Flyers were eliminated in Game 7.
Once the 2004–05 NHL lockout came to an end, Gagné assumed a greater role on the Flyers and played most of the year as the left winger on the top line with Peter Forsberg and Mike Knuble. The line became known as the "Deuces Wild" line due to their numbers (12 [Gagné], 21 [Forsberg] and 22 [Knuble]) and Gagné responded with his best season as a pro with 47 goals and 79 points in 72 games. He was awarded the Bobby Clarke Trophy, which is awarded to the Flyers Team MVP. In the off-season, Gagné signed a five-year contract extension worth over $25 million. Despite not having Forsberg as his centre for much of the 2006–07 season due to injury and Forsberg's eventual trade to the Nashville Predators, Gagné scored 41 goals and 27 assists in 76 games and won his second consecutive Bobby Clarke Trophy. He also played in his second NHL All-Star Game.
In the 2007–08 season, Gagné recorded 18 points in 25 games before being shut down for the year on February 20 after being told to sit out by doctors. Although it was believed Gagné had suffered three concussions in five months, he was told by doctors that he only had one and further injuries re-aggravated the symptoms. He suffered the initial concussion on October 25, 2007, when he was hit in the jaw with a check from Florida Panthers defenceman Jay Bouwmeester. Gagné sat out four games and returned only to be hurt again on November 7. After missing the next 26 games, he was re-injured on his first shift back on February 10, 2008, against the Pittsburgh Penguins.  Gagné follows after several previous Flyers player who have suffered serious concussions. Former captain Keith Primeau had his career ended prematurely due to concussions suffered while playing in Philadelphia, while Jeremy Roenick nearly retired after suffering one himself.
Gagné had a strong return in the 2008–09 season, scoring 34 goals, 74 points and posting a +21 in 79 games. However, injury troubles persisted in 2009–10.  Less than a month into the season, he was placed on the injured reserve after being diagnosed with a double hernia in his groin in late-October 2009. Although the Flyers' orthopedic surgeon recommended surgery, Gagné opted for a second opinion with his personal doctor in Montreal. Soon after, Gagné opted for surgery, which was performed on November 3. The surgery repaired the two small hernias in his right groin laparoscopically through his belly button, while reinforcing his rectus abdominis on both sides. Gagné returned to action on December 19 against the New York Rangers after missing 24 games. He scored his third career hat trick and first career natural hat-trick just 11 days after returning from injury against the Rangers, December 30, 2009. He finished the season with 40 points in 58 games. After defeating the second-seeded New Jersey Devils in the opening round of the 2010 playoff
Simon Gagné