He is called the father of Greek water polo and he is the presently the longest serving president of an athletic federation in Europe.
In 1965 he entered the NUOA Law School of Athens and immediately he registered in Center's Union Party Youth branch.
Two years later he was elected president of the Law School Students Union, just days before the beginning of the Greek military junta of 1967–1974.
During his presidency of the Hellenic Swimming Federation (KOE) he worked hard to prevent attempts by the government to reduce its economic and moral support of sports, and he is considered a key player in the struggle against doping in Greece.
The affair ended with Foura's expulsion from office; Diathessopoulos was widely recognized for his efforts, which have discouraged any further attempts by the state to cut sports financing.
With his protests, political initiatives, work on sports law, and even lawsuits against the Greek state, he has made an important contribution to the enactment of pro-sports developmental legislation.
In 2000 he was proposed for the office of General Secretary of Sports in Greece under the Ministry of Culture, but he rejected the offer, as he preferred to pursue a career within international swimming organizations.
His son Michael (Michail) Diathessopoulos, also an attorney at law and a Cambridge scholar, follows the steps of his father, with involvement in politics.