Johansson has played in a number of roles up and down the lineup throughout his NHL career since being selected by the Washington Capitals with the No. 24 pick in the 2009 NHL Draft. He returned to the Capitals via a trade with the Seattle Kraken on March 21, 2022, and played his 800th NHL game Jan. 17, 2023, a 4-2 loss to the Minnesota Wild. He was traded to the Wild on Feb. 28, 2023, for a third-round pick in the 2024 NHL Draft and signed a two-year, $4 million contract with Minnesota on May 2.

Johansson signed a one-year, $800,000 contract with the Wild on June 2, 2025, in lieu of becoming an unrestricted free agent. His assist on Marcus Foligno's goal at 18:29 of the third period that helped Minnesota to a 4-3 overtime win at the Detroit Red Wings on Feb. 22 was his 500th NHL point. Following the Wild's five-game loss to the Vegas Golden Knights in the Western Conference First Round, Johansson had eight points (four goals, four assists) in nine games to help Sweden to a bronze medal at the 2025 IIHF World Championship.

Johansson had an assist in his 1,000th NHL game, a 2-0 win for the Wild against the Calgary Flames in St. Paul, Minnesota, on Nov. 9, 2025. He scored his 200th NHL goal in a 5-2 victory against the Seattle Kraken on April 7, 2026.

When Johansson was selected by the Capitals, he had been playing as a wing with Farjestad of the Swedish Hockey League. But in 2009-10, he moved to center and saw his playing time and role increase, finishing with 20 points (10 goals, 10 assists) in 42 games.

The following season, Johansson earned a spot on the Capitals roster and made his NHL debut in a 4-2 loss to the Atlanta Thrashers on Oct. 8, 2010. He scored his first NHL goal in a 3-1 loss to the Boston Bruins on Oct. 19, and finished his rookie season with 27 points (13 goals, 14 assists) in 69 games.

Johansson would get at least 44 points in each of his next three full seasons. His flexibility to play different roles aided the Capitals in 2015-16, when he filled in for Jay Beagle while the forward two months with a broken hand.

Though Johansson had NHL career high in goals (24), points (58) and plus-minus (plus-25) in 2016-17, the Capitals' surplus of talent up front made him expendable and he was traded to the New Jersey Devils on July 2, 2017.

During his first season in New Jersey, Johansson missed all but 29 games because of injuries, and after getting 27 points (12 goals, 15 assists) in 48 games the following season, he was traded to the Bruins on Feb. 25, 2019.

With Boston, he played a key role in the 2019 Stanley Cup Playoffs, getting 11 points (four goals, seven assists) in 22 games to help the Bruins advance to the Final, where they lost to the St. Louis Blues in seven games.

Following the season, he signed a two-year contract with the Buffalo Sabres.

Internationally, Johansson also won a silver medal with Sweden at the 2014 Sochi Olympics. He was named to Team Sweden's roster for the Olympic Winter Games Milano Cortina 2026 as an injury replacement to Anaheim Ducks forward Leo Carlsson.