Kostandinos Georgiou Simitis was born in Athens on 26 January 1936. He is the son of George Simitis, an attorney and professor. He is married to Fani Arkadiou and has two children.
Simitis received his education in Germany and Britain. He studied law and economics at the University of Marburg in the Federal Republic of Germany (West Germany) between 1954 and 1959, where he earned his doctorate in 1959. From 1961 to 1963, he studied at the London School of Economics and Political Science.
After completing his studies, he worked as an attorney and taught commercial law at several universities. In 1971 he was assistant professor at the University of Konstanz in Germany. He served as a full professor of commercial law and civil law at the Justus Liebig University in Giessen during 1971–75. Since 1977 he has taught commercial law at the Pantions University of Political Sciences. He is the author of several books on political science and law and has also published numerous articles in law reviews.
In 1965 he was one of the founders of the Alexandros Papanastasiou Society (Society for Political Research and Studies). During 1967–69 he undertook clandestine actions against the junta but avoided arrest by escaping abroad. In 1970 he served as a member of the National Council of the Panhellenic Liberation Movement (PAK). During 1969–74 he stayed in Germany and participated in public meetings in opposition to the Greek dictatorship.
On 3 September 1974, the PASOK (Panhellenic Socialist Movement) was founded. Simitis was one of the founding members and served on both the central and the executive committees. He served as minister of agriculture between October 1981 and 1985 and minister of national economy from June 1985 to October 1987. He was elected to Parliament four times beginning in 1985. From June 1989 to November 1989, he was minister of education. After the October 1993 elections, he became minister of industry, energy, technology, and commerce.