Rakell was 19 when he made the Anaheim Ducks' opening-night roster for the 2012-13 season. Anaheim's first-round pick (No. 30) in the 2011 NHL Draft averaged 8:57 of ice time in four games before the Ducks returned him to Plymouth, his junior team in the Ontario Hockey League.

He split the 2013-14 season between Anaheim and Norfolk of the American Hockey League, playing in 18 NHL games and earning his first point with an assist in a 6-2 victory against the Buffalo Sabres on Nov. 8, 2013. Rakell made his Stanley Cup Playoff debut April 23, 2014, when he dressed for Game 4 of the Western Conference First Round against the Dallas Stars. He scored his first NHL goal in Anaheim's 6-2 victory in Game 5 and finished with a goal and an assist in four playoff games.

Rakell, still officially a rookie for the 2014-15 season, scored his first regular-season goal in Anaheim's 3-2 overtime win against the San Jose Sharks on Dec. 22, 2014. He tied a Ducks rookie record with four points (two goals, two assists) in a 5-4 win against the Winnipeg Jets on Jan. 11, 2015, and finished the season tied for ninth among all NHL rookies with 22 assists. He had one point, a goal, in 16 playoff games during Anaheim's run to the Western Conference Final.

In 2015-16, Rakell was used primarily as Anaheim's third-line center, although he also spent some time as the left wing on the top line with center Ryan Getzlaf and right wing Corey Perry and looked comfortable playing either position. He finished tied for third on the Ducks with 20 goals, including a team-high seven game-winners, was fourth with 43 points and third among Anaheim forwards with 121 hits. He had a goal, an assist and 13 shots on goal during Anaheim's seven-game loss to the Nashville Predators in the first round.

Rakell found his scoring touch in 2016-17, finishing with a Ducks-high 33 goals and 51 points in 71 games, then had 13 points (seven goals, six assists) in 15 playoff games. He led Anaheim in goals (34) and points (69) in 2017-18, including his first NHL hat trick in a 6-5 shootout loss to the Edmonton Oilers on Feb. 25, 2018.

Team Sweden added Rakell as an injury replacement for the World Cup of Hockey 2016. He was a member of Sweden's gold medal-winning team at the 2012 IIHF World Junior Championship, where he had five points (two goals, three assists) in six games. In 2013, he had six assists to help Sweden win the silver medal.

Rakell was traded to the Pittsburgh Penguins on March 21, 2022, and signed a six-year $30 million contract July 11. He represented Sweden at the 4 Nations Face-Off and played for Team Sweden the Olympic Winter Games Milano Cortina 2026. He was named the NHL First Star for the week ending April 5. He scored seven of the Penguins' 30 goals over five games, the most Pittsburgh scored in a five-game span since 30 from March 16-26, 1996.