2010-11: Ben Thomas skated for the Calgary Northstar Sabres bantam AAA team and played for the Calgary North Stars in the 2011 Alberta Cup. He scored 1 goal with 3 assists and had 2 penalty minutes in five games at the Alberta Cup. Thomas was selected by the Calgary Hitmen in the third round (45th overall) in the 2011 WHL Bantam Draft.
2011-12: Thomas made the jump from bantam hockey to midget majors — skating in the Alberta Midget Hockey League with the Calgary Northstars. In 34 regular season games he scored 2 goals with 15 assists and had 46 penalty minutes. The Northstars finished third in the Chrysler Division. Thomas did not appear in any playoff games.
2012-13: Thomas skated in three pre-season games with the Hitmen before playing junior A hockey in Calgary and made his WHL debut with Calgary in the playoffs. He had no points nor penalty minutes in his only game with the Hitmen. Thomas began the season with the AJHL's Calgary Mustangs — skating in 12 games and scoring 1 goal with 4 penalty minutes — before a mid-season trade to the Calgary Canucks. In 32 games with the Canucks he scored 4 goals with 3 assists and had 33 penalty minutes. The Canucks finished last in the South Division.
2013-14: Thomas spent his first WHL season with the Calgary Hitmen. He was selected to play for Team Orr in the 2014 CHL/NHL Top Prospects Game.