Athanasiou was selected by the Detroit Red Wings in the fourth round (No. 110) of the 2012 NHL Draft after scoring 59 points (33 goals, 26 assists) in 120 games in the Ontario Hockey League for his hometown team, London.
Prior to the 2012-13 season, London traded Athanasiou to Barrie. He broke out for 67 points (29 goals, 38 assists) in 66 games that season and helped Barrie advance to the OHL finals. Athanasiou finished with 25 points (12 goals, 13 assists) in 22 playoff games.