Dimas was born in Himara, Albania to parents of Greek origin, and emigrated to Greece in 1991. He first competed for Greece in the 1992 Summer Olympics, winning the gold medal in the 82.5kg class. His birthplace gave rise to his nickname "The Lion of Himara". His outstanding number of Olympic Gold Medals created another nickname "Midas".
With the 1993 and 1995 World titles under his belt, Dimas was heavily favoured for the 83kg class at the 1996 Summer Olympics, where he was the flagbearer of the Greek Olympic team. He won the event with two new World Records.
At the Sydney Olympics, he won yet another gold medal, this time in the 85kg class. This made him one of just three weightlifters to have won three Olympic gold medals, the others being Naim Süleymanoğlu (Turkey, but born in Bulgaria), Kakhi Kakhiashvili (Greece, but born in Georgia) and Halil Mutlu (Turkey).
At the 2004 Summer Olympics in Athens, Greece, Dimas was again chosen as Greek flagbearer for both the opening and closing ceremonies. Dimas was recovering from knee surgery and a hurt wrist and was not expected to compete in these Olympics, but it was the Olympics in Greece and national heroes rise to the challenge.
The situation in Greece was tense after the Greek sprinter steroid controversy. The crowd packed the weightlifting arena expecting a miracle and, considering the age and health of Dimas, they got one. He came away with a bronze medal in the 85kg class, becoming the first weightlifter in history to win a medal at four successive Olympic Games and cementing his status as a national hero in Greece. The unexpected 10-minute tribute/victory celebration swallowed up the gold and silver medalists and Dimas retired a national hero.