Kostas Tsimas was born in Theologos, on the north Aegean island of Thasos, on June 4, 1936 He was the son of a union tobacco worker. He lived in the United States for many years and was very active both politically and in the business sector. He was a New York University Political Science and Public Administration graduate.
He was a founding member of PAK (Pan Hellenic Liberation Movement) and a member of the National Council of PAK. He also was a founding member of PA.SO.K (Pan Hellenic Socialist Movement) and a close associate of Andreas Papandreou, with whom he returned to Greece after the fall of the dictatorship in 1974 and served as Hellenic National Intelligence Service head under his government.