2009-10: Andreas Johnson skated in five games for Frolunda's U18 team as a 16-year-old (his older brother Jonathan was the team's leading scorer) . He spent the bulk of the season at the U16 level and played for Goteborg 1 in the TV-Pucken tournament for Sweden's high school age players. Johnson scored 1 goal with 1 assist and had an even plus/minus in five U18 games. In eight games with Frolunda's U16 team he scored 11 goals with 6 assists and 6 penalty minutes. In eight TV-Pucken games he scored 9 goals with 4 assists and had 10 penalty minutes. Goteborg 1 captured the bronze medal.
2010-11: Johnson had a breakout offensive season skating for Frolunda's U20 and U18 teams as both clubs captured gold medals at their respective levels. He scored 9 goals with 5 assists and was +5 with 4 penalty minutes in 30 games for Frolunda's U20 team. In three playoff games he was an even plus/minus with 1 assist. (Jonathan Johnson spent most of the season with Frolunda's U20 team but joined the U18 team for the playoffs). Andreas Johnson was the leading scorer for the Frolunda U18 team with 23 goals with 22 assists in 27 games and was +41 with 26 penalty minutes. Johnson scored 3 goals with 1 assist and was +1 with 2 penalty minutes in four playoff games.
2011-12: Johnson spent most of the season with Frolunda's U20 team, returning to the club's U18 squad for its bronze medal playoff run. He scored 19 goals with 13 assists and was +29 with 54 penalty minutes in 42 games as Frolunda finished ninth in the SuperElit Top 10 and lost to Linkopings in the first round. Johnson was -3 with no points or penalty minutes in two playoff games. Playing for the Frolunda U18 team he scored 9 goals with 5 assists and was +13 with 4 penalty minutes in six regular season games and scored 2 goals with 4 assists in six playoff games.
2012-13: Johnson made his pro debut in Sweden's Elitserien, playing 12 games with Frolunda, and was the leading scorer for the club's U20 team. He also represented Sweden internationally. Johnson made his elite league debut in December, scoring his only goal in his second game, during a six-game stretch with the team. He re-joined Frolunda for one late season games and skated in 5 of 6 games in the playoffs. He averaged just under four minutes ice time in the regular season games and was scoreless in the playoff series against Lulea. Johnson scored 23 goals with 31 assists and was +29 with 54 penalty minutes in 42 regular season games for Frolunda U20, which finished first in the regular season before falling in the playoff quarterfinals. Johnson scored 1 goal with 1 assist and was +3 with 12 penalty minutes in four playoff games. In four games for Sweden's U19 team he scored 2 goals with 3 assists and was +2 with 8 penalty minutes. Ranked 33rd amongst European skaters, Johnson was selected by Toronto in the seventh round (202nd overall) of the 2013 NHL Draft.