[1] After playing college basketball for the Florida Gators, Calathes won the EuroLeague title with Panathinaikos in 2011, and reached another final in 2021 with FC Barcelona.

Calathes represents the Hellenic national basketball team internationally, and won a bronze medal at EuroBasket 2009.

He was a first-team All-Southeastern Conference (SEC) selection in 2009, and he was the only player in the nation to average better than 15.0 points, 5.0 rebounds, and 6.0 assists per game.

In his first year with Panathinaikos, he won the Greek League championship, averaging 5.4 points, 2.0 assists, and 2.2 rebounds in 15 minutes per game.

Barcelona head coach Xavi Pascual said, "Diamantidis was the MVP of the series, but Calathes was the key player."

In the four-game playoff series against Barcelona, Calathes averaged 7.8 points, 2.5 rebounds, 2.3 assists, and a steal in 26 minutes per game.

At the 2011 EuroLeague Final against Maccabi Tel Aviv, Calathes helped Panathinaikos, by scoring four points, recording six assists, and making two steals.

In the summer of 2012, Calathes signed a two-year contract with the Russian VTB United League team Lokomotiv Kuban Krasnodar.

On July 22, 2013, the Dallas Mavericks traded Calathes's draft rights to the Memphis Grizzlies in exchange for a 2016 second-round pick.

[10] In February 2014, starting for the injured Mike Conley Jr., he was named Rookie of the Month, averaging 10.7 points, 4.6 assists and 3.6 rebounds per game.

The NBA statement announcing the suspension, which took effect immediately, also indicated that the banned substance was an ingredient in a supplement that Calathes had been taking.

On October 22, against the Turkish Super League club Karşıyaka Basket, Calathes set a new personal career-high of 11 assists in a EuroLeague game.

[19] On June 23, 2022, Calathes was excluded from the plans of Barcelona coach, Šarūnas Jasikevičius, for the 2022–23 season,[20] after arguments between the two ex-teammates and due to the decision of the Blaugrana to cut down on their budget.

On August 23, 2022, Calathes signed a two-year contract with Fenerbahçe of the Turkish Basketball Super League (BSL).

[21][22] In a highly successful 2023–24 season, which saw Calathes reunite with his former coach Jasikevičius, Fenerbahçe made its return to the EuroLeague Final Four after a five-year break.

[24] Calathes made his debut with the Greek junior national team at the 2008 FIBA Europe Under-20 Championship, where he averaged 11.2 points, 3.5 rebounds, and 3.7 assists in 22 minutes per game.

In the semifinal against Spain, despite his team losing the game, he scored 10 points, grabbed four rebounds, dished two assists, and made two steals.