During the 2009–10 season, 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. Athanasiou recorded 24 goals and 34 assists in 56 games. Athanasiou was drafted by London 81st overall in the 2010 OHL Priority Draft.
During the 2010–11 season, Athanasiou skated in 57 games as an OHL rookie with the London Knights. Athanasiou recorded 11 goals and 11 assists. 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.
During the 2011–12 season, 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. Athanasiou recorded 22 goals and 15 assists in 63 games. Athanasiou skated in nine of the Knights' first 17 playoff games as London reached the conference finals and recorded one goal and 3 assists. Athanasiou 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.
During the 2012–13 season, Athanasiou skated for the Barrie Colts in his third OHL season after being obtained by the Colts in an August 2012 trade with London. Athanasiou was the fourth-leading scorer for the Colts, finishing third on the team with 29 goals and recording 38 assists in 66 games. Barrie reached the OHL championship against Athanasiou's former team. In 22 playoff games Athanasiou recorded 12 goals and 13 assists. Athanasiou joined the Detroit Red Wings' AHL affiliate Grand Rapids Griffins during the Calder Cup playoffs but did not appear in any games.
During the 2013–14 season, Athanasiou led Barrie in scoring, and ranked fifth in the league in scoring during his final OHL season, recording 49 goals and 46 assists in 66 games.
On November 20, 2013, the Detroit Red Wings signed Athanasiou to a three-year, entry-level contract.
On April 15, 2014, Athanasiou was assigned to the Grand Rapids Griffins, and made his AHL debut on April 18, in a game against the Lake Erie Monsters.
During the 2014–15 season, in his first full professional season, he posted the best point-per-game average, 0.58, among Griffins rookies, recording 16 goals and 16 assists in a campaign that was limited to 55 games played due to a broken jaw. He finished fifth on Grand Rapids in postseason scoring, recording five goals and four assists in 16 postseason games.
Athanasiou made his NHL debut for the Red Wings on November 8, 2015 in a game against the Dallas Stars. On November 10, in his second NHL game, Athanasiou scored his first career NHL goal against Braden Holtby of the Washington Capitals. On February 5, 2016, Athanasiou was recalled by the Detroit Red Wings. Prior to being recalled, he recorded eight goals and eight assists in 26 games for the Griffins.
On April 30, 2016, Athanasiou was assigned to the Grand Rapids Griffins. During the 2015–16 season, Athanasiou recorded nine goals and five assists in 37 games for the Red Wings, while averaging 9:01 of ice time.
Athanasiou was part of the gold medal-winning Ontario team at the 2011 World U-17 Hockey Challenge. Athanasiou was one of five players for Ontario who had five or more points in the five-game WHC, finishing with two goals and three assists.
Athanasiou is ethnically Greek; his last name originates on the Greek island of Samos.