2009-10: Thorell made his pro debut as a 17-year old, skating in four games for Farjestad and also playing for Skare BK in Division I (Sweden's third level). He played for both clubs' junior teams and represented Sweden in the 2010 U18 World Junior Championship. He was scoreless for Farjestads, averaging just under seven minutes of ice time. In 17 games for Skare BK he scored 10 goals with 10 assists and was plus-nine with 14 penalty minutes. In 17 games with Farjestad's U18 squad he scored 19 goals and 38 assists and had 18 penalty minutes. In four playoff games he scored 4 goals with 2 assists. Thorell had five points in his only game with Skare BK's U20 team – scoring 2 goals with 3 assists. In six games for silver medal-winning Sweden at the 2010 U18 WJC he scored 3 goals and was minus-two with 4 penalty minutes. Thorell was ranked 40th amongst European skaters in Central Scouting's final rankings prior to the NHL Draft in his first year of eligibility but was not selected.
2010-11: Thorell appeared at three different levels of pro hockey in Sweden and represented Sweden at the 2011 U19 World Junior A Challenge. Thorell played three games for Farjestad during the regular season and was with the club during its championship playoff run. He averaged just over 2 minutes ice time in three regular season games and had just over 2 minutes of ice time total while dressing for five games during the playoffs. In 22 games for Sundsvall in Allsvenskan (Sweden's second league), he scored 2 goals with 8 assists and had 10 penalty minutes. He played in 17 games for Skare BK in Division 1 and had 11 goals with 11 assists and 56 penalty minutes. In four games at the WJAC he had no points and 6 penalty minutes. Thorell was not among the 140 European skaters listed in Central Scouting's final rankings prior to the 2011 NHL Draft and was not selected in his second year of draft eligibility.
2011-12: Thorell skated in 44 games in his first full season with Farjestad and skated for Sweden's gold medal winning squad at the 2012 U20 World Junior Championship. He scored 3 goals with 3 assists and was minus-2 with 2 penalty minutes – averaging 8:29 minutes of ice time for Farjestad. After finishing sixth in the 12-team Elitserien, Farjestad defeated HV71 in the first round of the playoffs before falling to eventual champion Brynas. In 11 playoff games Thorell had 1 assist and was minus-two with 4 penalty minutes. At the WJC he was plus-five with 3 goals and 3 assists as Sweden won the championship; defeating Russia, 1-0 in the championship game. Thorell was ranked 19th amongst European skaters in Central Scouting's final rankings prior to the 2012 NHL Draft.