Mattias Falck

2021 In March, Karlsson was upset by Dimitrij Ovtcharov in the quarter-finals of the WTT Contender event at WTT Doha. He was upset in the round of 32 by An Jaehyun in the WTT Star Contender event.

In June, Karlsson reached the semi-finals of the 2021 European Table Tennis Championships, where he was upset in the semi-finals by eventual champion Timo Boll.

Karlsson had a disappointing 2020 Olympics, losing in the round of 16 in the singles event to Omar Assar. In the team event, Sweden defeated USA in the first round 3–1, but Falck was upset by Kanak Jha. Sweden lost to Japan in the quarter-finals after Koki Niwa upset Karlsson in the fourth round. Niwa later stated that Karlsson's forehand was not in good condition.

Mattias and Kristian Karlsson became World champions in the men's doubles at the 2021 World Table Tennis Championships becoming the first Swedish duo to win gold since 1991.

2023 In August, Karlsson won WTT Contender title at Rio de Janeiro after defeating Sora Matsushima from Japan in 4-2 set score and winning the WTT title for the very first time. In September, Karlsson was playing in the Swedish National team that won the 2023 STUPA European Table Tennis Championships in Malmö. That victory marked Sweden's first team title win in 21 years. The team also consisted of Truls Moregardh, Kristian Karlsson and Anton Kallberg. Jorgen Persson was head coach.

Playing style Karlsson plays with short pips on his forehand. He has stated that his style benefited from the switch to the plastic ball and that he was surprised that more men with pips have not emerged since the ball switch.

Personal life In 2018, Mattias adopted his wife's surname, Falck, upon marriage. He resumed using his birth name, Karlsson, following their divorce in 2026.