Giannakis was born on January 1, 1959 in Nikaia, Piraeus. He started his playing career at local team Ionikos in 1971, transferred to Aris in 1984, played for Panionios in 1994) and for Panathinaikos in 1995 and 1996, finishing his career with a total of 9,291 points which places him 3rd on the all-time scoring list.
As a player, Giannakis won:
- the A1 championship 7 times (all with Aris)
- the Greek Basketball Cup 7 times (6 with Aris, once with Panathinaikos)
- the European Cup-Winners Cup once (with Aris in 1993)
- the European Champions' Cup (Euroleague) once in 1996 with Panathinaikos.
Giannakis has participated in 5 Euroleague final-fours (three with Aris, twice with Panathinaikos).
During his playing career, Giannakis played 351 times for the National Basketball Team of Greece (a European record for caps) scoring a total of 5,291 points (a Greek record). Besides leading Greece to Gold in 1987, he led them to silver in Eurobasket 1989 and to 5th place in the Atlanta Olympiad of 1996.
After his playing days were over, Giannakis became the coach of the Greek National Team. He made his debut on June 4, 1997 against Israel with a 68-62 victory but the high point of his career came in his second tenure as National Team coach, on September 25, 2005, when Greece defeated Germany 78-62 to win Eurobasket 2005. In between, Giannakis coached Panionios and Marousi.
Giannakis is married to Eugenia with two children: Calliope and Dimitris.