[4][5] Born in Trollhättan, Västra Götaland, Karlsson started to play table tennis in his hometown club at the age of 8.

[citation needed] In 2011, he signed to Halmstad BTK and began to rise through the ranks quickly.

[8] The year 2012 also marked his first senior success, winning the silver medal at the 2012 Table Tennis European Championships in men's doubles.

[9] His performances also attracted French top division side AS Pontoise-Cergy TT and Karlsson eventually signed to the club in September 2013.

In the second, WTT Star Contender event, Karlsson upset Liam Pitchford in a deuce-in-the-fifth thriller in the round of 32.