In 1948 he graduated from the Hellenic Naval Academy and later continued his studies in other academies, including taking the Senior Officers' War Course at the Royal Naval College, Greenwich.
During the Greek military junta of 1967–74, he was opposed to the regime, like many Navy officers.
In January 1982 he was appointed as chief of the Hellenic National Defence General Staff by the government of Andreas Papandreou, as he was one of the officers considered most trustworthy by the newly elected government of PASOK.
In October 1989 he became Minister for National Defence in the Caretaker Cabinet of Ioannis Grivas.
[3] His brother was Ioannis Degiannis, the presiding judge at the Greek Junta Trials and member of parliament with PASOK.