2011-12: Miro Karjalainen skated for the Espoo Blues second team at the U16 level. In 23 games he had 7 assists and 57 penalty minutes.
2012-13: Karjalainen played for the lower division club EKS in Espoo — skating in two game’s with the club’s Division III men’s team — and appeared in two games with the Kiekko-Vantaa U18 team in the top division. He had no points nor penalties playing for the EKS men’s team nor with Kiekko-Vantaa. In four games with the top EKS U18 team he scored 4 goals with 2 assists and 2 penalty minutes.
2013-14: Karjalainen went from Espoo to Helsinki to skate for the Jokerit U18 team and was a steady force on the blueline as the team captured the B juniors national title. He scored 1 goal with 7 assists and had 57 penalty minutes in 38 regular season games and in 11 playoff games scored 2 goals with 3 assists and 6 penalty minutes. Karjalainen was ranked 139th amongst the 140 international skaters in the Central Scouting final rankings and was selected by Dallas in the fifth round (135th overall) of the 2014 NHL Draft.