Johnsson played with the Toronto Maple Leafs, New Jersey Devils, San Jose Sharks, and Pittsburgh Penguins in the National Hockey League (NHL).

[3] On 21 April 2014, Johnsson was named the SHL Rookie of the Year, beating out Frölunda teammates Alexander Wennberg and fellow Maple Leafs prospect Tom Nilsson, as well as HV71's Kevin Fiala.

[5] The following season was another sensational year for Johnsson, in which he finished sixth in league scoring and helped Frölunda capture their fourth championship.

[6] However, his stint was cut short, as in his second North American game, Johnsson was hit in the head by an elbow from Albany Devils player Dan Kelly.

[8] During the 2017–18 season, Johnsson was named to the 2018 AHL All-Star Game as a replacement for fellow Maple Leafs prospect Kasperi Kapanen, who was called up to the NHL.

[15] After leading all players in points during the playoffs, Johnsson was awarded the Jack Butterfield Trophy as MVP of the Calder Cup.

[16][17] As a restricted free agent entering the off-season, Johnsson accepted his qualifying offer from the Maple Leafs, signing a one-year, two-way contract worth $787,500.

[26] Signalling the end of his tenure in North America, Johnsson immediately returned to his native Sweden and rejoined the SHL by agreeing to a four-year contract with Skellefteå AIK on 10 November 2023.