2010-11: Anton Karlsson appeared in four games with the Frolunda U18 team as a 14-year-old and played for Goteborg 2 in the annual TV-Pucken tournament for Swedish high school age players. He had 1 assist with the Frolunda U18 team. Karlsson scored 7 goals with 1 assist in eight TV-Pucken games.
2011-12: Karlsson spent most of the year with Frolunda's U18 team and played for Sweden's U16 and U17 teams in international play as well as skating for Goteborg in the TV-Pucken tournament. Karlsson scored 12 goals with 19 assists in 28 U18 games and was +20 with 32 penalty minutes. Frolunda won the U18 bronze medal. In three matches with Sweden's U17 team Karlsson scored 2 goals with 1 assist and he scored 2 goals with 6 assists in eight games with Sweden's U16 team. He scored 6 goals with 6 assists in eight TV-Pucken games.
2012-13: Karlsson played for Sweden's U18 team in the 2013 Ivan Hlinka Memorial Tournament in August before returning to Frolunda — where he split the season between the U20 and U18 squads. He later represented Sweden in the 2013 U17 World Hockey Challenge, where he was the team captain, and at the 2013 U18 World Junior Championship. He played 20 games in Sweden's SuperElit U20 league, including three playoff games, and scored 5 goals with 5 assists; finishing +7 with 4 penalty minutes. In 23 games for Frolunda's U18 team he scored 20 goals with 19 assists and was +12 with 64 penalty minutes. In three U18 playoff games he scored 1 goal with 3 assists and was +2. Karlsson had 2 assists and was +3 in five games at the Ivan Hlinka Tournament and was an assistant captain for the U18 team at the WJC. He scored 2 goals with 2 assists and was +3 with 10 penalty minutes in five games at that tournament. Sweden's U17 team won the gold medal at the WHC and Karlsson scored 6 goals with 3 assists and 22 penalty minutes in six games.
2013-14: Karlsson skated in his second straight Ivan Hlinka Tournament in August — serving as a team captain for Sweden's seventh-place squad before returning to Frolunda's U20 team. In four games at the Ivan Hlinka Tournament he scored 1 goal with 4 assists.