He started his senior career at the age of just 13 at Ionikos Nikaias and after noticeable success, he achieved extraordinary success as a player with Aris Thessaloniki from 1984 to 1993, in partnership with Greek basketball's biggest star Nikos Galis along with the rest of the squad, while achieving victory as the national team captain in EuroBasket 1987 in Athens, Greece's first major tournament win in non-Olympic sport which, along with Aris' extraordinary success, cultivated the sport in the country.

At the end of his career, he won the EuroLeague with Panathinaikos in 1996 As head coach, he most prominently led Greece to its second European trophy in EuroBasket 2005 at Serbia and Montenegro, where a team of new and talented stars shone brightest under the nurturship of Giannakis.

He would repeat the success the next year, acquiring the silver medal at the 2006 FIBA World Championship in Japan, where the team most notably scored a decisive win against the USA stars of the NBA in the semi-final, which would be USMNBT's last loss in a tournament until 2019.

[8] A true floor general, Giannakis began his club basketball playing career in Greece, with Ionikos Nikaias.

After that, he before moved to the Greek club Aris Thessaloniki, where he spent the most important part of his pro playing career.

After his playing career ended, Giannakis was the head coach of the Athenian professional club team, Maroussi Athens, which he led to the forefront of the Greek League.

Ηis father Dimitris, hailed from Asia Minor, and was an owner of a motorbike workshop; his mother Kalliope, was a weaver.

His first head coach, George Vassilakopoulos, moved Giannakis up to the club's senior men's first team, from the youth squad, in 1972, when he was at the age of just 13.

Giannakis' exceptional play with Ionikos Nikaias, at such a young age, drew the eyes of pro basketball experts on him.

At that time, the Boston Celtics used the facilities of the Hellenic College's campus in Brookline, Massachusetts, for their training camp and practices.

[13][14] While at training at Hellenic College, Giannakis was eventually noticed by the Celtics, and they invited him to join the team's 1981 summer league camp.

[16] However, after ultimately recovering from his knee surgery, Giannakis went on to average 25 points per game with the Hellenic College Owls, during the 1981–82 season.

Giannakis also personally received a BMW car, a sporting goods store, and an 8 million drachmas signing bonus from Aris.

With Aris, Giannakis teamed up with Nikos Galis, to form one half of an historic "tag-team", that took both Greek and European basketball by storm, for the years to come.

From the 6th of March 1985, to the 5th of November 1988, the backcourt duo of Galis and Giannakis, led Aris Thessaloniki to an 80-game winning streak in the Greek League.

Aris Thessloniki joined the elite of European basketball clubs at that time, but a European-wide title did not come for Giannakis and his team until a few years later.

The 1991 FIBA Jubilee event was held in order to commemorate the 100th anniversary of the creation of the sport of basketball in 1891, by the Canadian James Naismith.

The FIBA Jubilee All-Star Game took place at the Peace and Friendship Stadium, in Piraeus, Athens, Greece, and it included numerous legends of European basketball.

Aris beat the Turkish Super League club Efes Istanbul, by a score of 50–48 in the final, which was held in Turin.

With Panionios Athens, he averaged 14.0 points, 3.6 rebounds, 3.3 assists, and 1.4 steals per game, in the Greek Basket League's 1993–94 season.

With Panathinaikos Athens, Giannakis finally won the championship of the top-level European-wide club competition, the FIBA EuroLeague.

Giannakis led the Greek under-16 junior national team to the silver medal at the FIBA Europe Under-16 Championship of 1975.

Giannakis also represented Greece at the 1991 FIBA Centennial Jubilee, which commemorated the 100th anniversary of the creation of the sport of basketball, by the Canadian James Naismith.

Giannakis holds the all-time career record for the most caps with the Greece senior men's national team, with 351.

Giannakis' salary with the Greece men's national team, eventually reached an annual rate of €1.2 million net income.

In December 2008, Giannakis ended his tenure as the head coach of Greece's senior national team.

During his time with Maroussi Athens, along with the help of his young star point guard, named Vassilis Spanoulis, whose playing style and player attributes were often compared to Giannakis', he took the club from relative obscurity, and turned it into the third most prominent team in the Greek League, during that era.