2009-10: Athanasiou scored at a better than a point-per-game pace playing for the Toronto Titans Midget AAA team in the Greater Toronto Hockey League. In 56 games he scored 24 goals with 34 assists and had 32 penalty minutes. He was selected by London in the fourth round (81st overall) of the 2010 OHL Priority Draft.
2010-11: Athanasiou skated in 57 games as an OHL rookie with the London Knights and was part of the gold medal-winning Ontario squad at the 2011 U17 World Hockey Challenge. He scored 11 goals with 11 assists and was minus-11 with 22 penalty minutes. The Knights reached the OHL playoffs despite finishing last in the Midwest Division. In six playoff games Athanasiou was minus-one with no points or penalties. Athanasiou was one of five players for Ontario who had five or more points in the five-game WHC, finishing with 2 goals and 3 assists.
2011-12: Athanasiou was one of five players to score 20 or more goals on a London Knights team that finished with the OHL's best record. In 63 games he scored 22 goals with 15 assists and was plus-22 with 22 penalty minutes. Athanasiou skated in nine of the Knights' first 17 playoff games as London reached the OHL playoff finals and scored 1 goal with 3 assists and was plus-six. He was invited to the NHL Draft Combine and ranked 41st amongst North American skaters in Central Scouting's final rankings prior to the 2012 NHL Draft.
2012-13: Athanasiou skated for Barrie in his third OHL season after being obtained by the Colts in an August 2012 trade with London. He was the fourth-leading scorer for the Colts, finishing third on the team with 29 goals and recording 38 assists. He was +13 with 30 penalty minutes in 66 games. Barrie reached the OHL finals against Athanasiou's former team. In 22 playoff games Athanasiou scored 12 goals with 13 assists and was +6 with 11 penalty minutes. He joined Detroit AHL affiliate Grand Rapids during the Calder Cup playoffs but did not appear in any games.
Athanasiou has the speed and hands to be a dangerous NHL forward, though his all-around game is still developing. He does have the physical skills and skating ability to turn into an offensive threat at the pro level as long as he continues to work on his defense and consistency.
Athanasiou will return to the OHL's Barrie Colts in 2013-14, where he will look to build on a successful 2012-13 campaign.