[12] At the age of fourteen, Diamantidis started his basketball career with the youth clubs of his hometown team, AS Kastoria.

[2] Amidst a long series of "derbies of the eternal enemies", opposing Panathinaikos and Olympiacos Piraeus, Diamantidis thrived, as he faced Olympiacos every year for twelve consecutive seasons, including in eleven Greek League Finals, six Greek Cup Finals, and a EuroLeague semifinal.

[15] As a member of the Greece men's national basketball team, Diamantidis won a EuroBasket gold medal in 2005; while anchoring a defense that allowed just 59.7 points per game.

In 2006, he played a pivotal role in a FIBA World Cup silver medal campaign, with Greece stunning Team USA in the semifinals, and Diamantidis finishing as the competition's steals leader.

[17] Diamantidis was also a two-time Olympian, as he donned the Greek colours in 2004 and 2008, and on both occasions earned the fifth place of the tournament, which is tied for the all-time best finish for Greece.

On the basis of his myriad achievements, he was named the Mister Europa Player of the Year by Italian sports magazine Superbasket in 2007.

In summer 1999, at age 19, Diamantidis began his professional career, when he signed with the Greek Basket League club Iraklis Thessaloniki.

Diamantidis also once again played in the Pan-European secondary league, in which he averaged 7.1 points, 5.2 rebounds, 2.8 assists, 1.7 steals, and 0.8 blocks per game, during the Saporta Cup's 2001–02 season.

He continued to improve his individual numbers and performances, as he averaged 11.5 points, 5.6 rebounds, 5.0 assists, a league-leading 2.5 steals, and 0.9 blocks per game, during the 2002–03 Greek League season.

After his continued improvement and success with Iraklis, Diamantidis made a big step up in his career, with his move to Panathinaikos of Athens, in 2004; a basketball powerhouse of the Greek Basket League, and also of the EuroLeague.

Due to great individual and team successes, the late pharmaceutical magnate Pavlos Giannakopoulos, who was at that time the President of Panathinaikos, signed Diamantidis in 2008, to a 3-year, €5.7 million net income contract extension.

[26] On 2 September 2015 Diamantidis announced his retirement from playing professional club basketball, effective at the end of the 2015–16 Greek League season.

[28] Diamantidis retired as the all-time career leader in both assists and Performance Index Rating (PIR) in the entire history of the EuroLeague.

While he was a member of Panathinaikos, Diamantidis and Željko Obradović shared one of the most successful collaborations between a player and his head coach in the history of European club basketball.

[31] As a part of the Greek men's under-26 national selection, Diamantidis won the silver medal at the 2001 Mediterranean Games.

Diamantidis made 7 points, 7 rebounds, 4 assists, and 2 blocks for Greece at the 2007 EuroBasket, losing the bronze medal to Lithuania, 78–69.

Four years later, in 2020, he became the general manager of the Greek club Panathinaikos Athens, the same team that he had spent the last 12 seasons of his playing career with.

He was primarily a pass first playmaker with outstanding court vision, and an excellent three point shooter, both in spot up situations and off the dribble.

As demonstrated by numerous gamechanging actions in crucial situations, Diamantidis was considered to be one of the best decision makers and clutch players in European basketball history.