Selected by his hometown Ottawa Senators in the 2011 NHL Entry Draft, Pageau began his professional career in 2012–13 with Ottawa's American Hockey League affiliate, the Binghamton Senators, registering 7 goals and 29 points in 69 games before being called-up to Ottawa on April 10, 2013. Pageau stayed in Ottawa for the remainder of the season, registering two goals and four points in nine regular season games. On May 5, 2013, he scored a hat-trick in Game 3 of the Eastern Conference Quarterfinals series as the Senators defeated the Montreal Canadiens 6–1. In so doing, he became just the second Senators player to score a hat-trick in the playoffs after Daniel Alfredsson. Following the season, Pageau was honoured by his hometown; on June 18, 2013, he signed the official guestbook at the start of the Gatineau City Council meeting and received a plaque. Mayor of GatineauMarc Bureau called him "a true ambassador for the city". Pageau would only appear in parts of the 2013–14 and 2014–15 seasons under head coachPaul MacLean. Upon MacLean being fired, new head coach Dave Cameron would allow Pageau to play more games during the 2014–15 season, including all six of the Senators' 2015 playoff games. In June 2015, it was announced Pageau signed a two-year, one-way contract with the Senators worth an annual average of $900,000. Pageau would play in all 82 games during the 2015–16 season and achieve career highs in goals, assists and points, and led the NHL in short-handed goals with seven. Pageau scored four goals in Game 2 of round two of the 2017 playoffs against the New York Rangers, including the game-winner in double overtime. On July 17, 2017, Pageau signed a new three-year, $9.3 million contract worth $3.1 million annually, avoiding arbitration. In the final year of his contract with the Senators in the 2019–20 season, Pageau emerged as the club's top line center, scoring at a career high pace with 24 goals and 40 points in 60 games.
With the Senators out of playoff contention and as a potential free agent, Pageau ended his eight-year tenure with Ottawa when he was traded to the New York Islanders at the NHL trade deadline in exchange for a conditional first-round pick and a second-round pick in 2020 and a conditional third-round selection in 2021 on February 24, 2020. He was subsequently signed to a six-year, $30 million contract extension to remain with the Islanders through 2026.
International play
On April 12, 2018, Pageau was one of the 18 players to be named to the 2018 IIHF World Championship to represent Canada, where he scored one goal and tallied three assists.
