Athinaiou first played professionally with the Greek clubs
Ilysiakos,
Panionios, and
KAOD. In the summer of 2009, he signed a 4-year contract with Panionios. He joined the
Greek League club
Aris Thessaloniki, in 2013. Athinaiou joined the
Italian League club
Olimpia Milano, in 2014. In May, he parted ways with the team. He signed with the Greek
EuroLeague team
Olympiacos Piraeus, in 2015. With Olympiacos, he won the Greek League championship in 2016. He returned to
Aris Thessaloniki, for the 2017–18 season. On July 29, 2018, Athinaiou signed a two-year contract with Aris' arch-rivals,
PAOK Thessaloniki. In January 2019, he joined the
French League club
Fos Provence. On August 12, 2019, Athinaiou moved to the Greek EuroLeague club
Panathinaikos Athens. After joining Panathinaikos, Athinaiou became the first player in history to be a member of all five of the largest Greek basketball teams, having been a member of Panathinaikos, Olympiacos, AEK, PAOK and Aris. However, just a few days later, Athinaiou unfortunately suffered a
torn ACL knee injury, in one of the
Greek national team's preparation games against
Serbia, just two weeks prior to the start of
2019 FIBA World Cup. As a result of the injury, Athinaiou missed the entirety of the following
COVID-19 pandemic shortened 2019–20 season. On July 16, 2020, Athinaiou signed with the Greek club
Peristeri. On December 17, however, he was released from Peristeri after the acquisition of
Vangelis Mantzaris. The following day, Athinaiou signed with
Promitheas Patras, in an unofficial "swap" between the two teams. On April 17, 2021, Athinaiou agreed to move to Spanish club
RETAbet Bilbao Basket for the rest of the season, replacing the injured
Jonathan Rousselle. On August 6, 2021, Athinaiou signed with
Ionikos. In 18 games, he averaged 9 points, 3.2 rebounds and 4.4 assists, playing around 25 minutes per contest. For the 2022-2023 season, Athinaiou moved to the newly promoted
Karditsa, where he was named team captain. In 21 league games, he averaged 4.3 points, 1 rebound and 2.9 assists in 19 minutes per contest. ==National team career==