2009-10: Thomas Spelling made his debut with the Herning Blue Fox in Denmark's top pro league as a 16-year-old after skating in seven games for the club's second team the previous season. He was the second-leading goal scorer for Denmark's U18 team. Spelling scored 5 goals with 9 assists and had 6 penalty minutes in 32 games for the Blue Fox. After finishing fifth in the nine-team AL-Bank Ligaen, Herning advanced to the playoff semifinals and defeated Fredrikshavn in the bronze medal game. In 10 playoff games, Spelling scored 2 goals and had 6 penalty minutes. Herning was the host site for the 2010 U18 Division I Group A World Junior Championship and in five games Spelling scored 6 goals with 1 assist and was plus-one as Denmark finished second behind Norway.
2010-11: Spelling skated in all 39 games for Herning in his second season with the Blue Fox and represented Denmark at the U18 and U20 levels in junior tournament play. He scored 18 goals with 11 assists and had 14 penalty minutes during the regular season. Herning finished second to Soenderjyske and then captured the playoff championship. In 11 playoff games Spelling scored 3 goals with 4 assists. In the 2011 U20 Division I Group B World Junior Championship he scored 3 goals with 4 assists and was plus-three with 14 penalty minutes in five games as Denmark captured the tournament to gain elevation into the main group for 2011-12. Denmark's U18 team also won it's tournament in Division I Group B and in five games Spelling was plus-six with 5 goals and 5 assists. He was ranked 128th amongst European skaters in Central Scouting's final rankings in his first year of draft eligibility but was not selected in the 2011 NHL Draft.
2011-12: Spelling was Herning's second-leading scorer despite missing seven games during the season and competed for Denmark in the 2012 U20 World Junior Championships. Spelling scored a team-leading 21 goals and had 16 assists with 6 penalty minutes in 33 games for the Blue Fox. Herning, after finishing third in the regular season, captured its second straight AL-Bank Ligaen championship; defeating Odense in the finals. Spelling skated in all 17 playoff games and was the team's leading scorer with 10 goals and 10 assists. Denmark proved to be overmatched in the top division, failing to win a game at the WJC, and in six games Snelling finished minus-11 with 1 goal and 3 assists. Ranked 90th amongst European skaters in Central Scouting's final rankings prior to the 2012 NHL Draft, Spelling was selected by the Rangers in the fifth round (142nd overall).
2012-13: Spelling went from Denmark to Sweden and made his SHL debut as a 19-year-old – skating in seven games for last-place Rogle. In limited ice time, he was -1 with no points nor penalty minutes. He also played four games in AllSvenskan with Oskarshamn, and was again scoreless. Spelling was more successful with Rogle's U20 team and playing with Denmark's U20 team in the 2013 Division 1 Group A World Junior Championship. He was the second-leading scorer for Rogle with 19 goals and 30 assists and was +28 with 6 penalty minutes for the sixth-place club. Spelling led third-place Denmark with 7 assists and had 1 goal with 2 penalty minutes in five games at the D1A WJC.