He previously played for the San Jose Sharks of the National Hockey League (NHL).

[2] During Karlsson's first tenure with Skellefteå AIK, he was a two-time Le Mat Trophy champion, winning the Swedish Championship in 2013 and 2014.

[4] After beginning the subsequent season with the Worcester Sharks of the American Hockey League, on 9 December 2014, it was announced that Karlsson would make his NHL debut that night in a game against the Edmonton Oilers.

[5] Karlsson scored his first NHL goal on 22 December 2014, against Frederik Andersen of the Anaheim Ducks.

On 15 December 2020, Karlsson opted to remain in his native Sweden, returning to his original club, Skellefteå AIK of the SHL, for the remainder of the season.