He completed his basic education at the Experimental School of the Aristotle University of Thessaloniki. He obtained his degree in History and Archaeology from the University of Thessaloniki in 1959 and completed his PhD thesis at the University of Munich in Germany in 1972 on «Fruhgriechische Grabbauten»,
In 1962 he was hired as a research assistant the Archaeological Services of the prefecture of Thessaloniki and took part in the excavations in Western Macedonia; especially in the archaeological site of Pella and the palace of Vergina under the direction of Haralambos Makarios, Photios Petsas and Manolis Andronikos. Between 1963 and 1980 he served as curator of antiquities at the archaeological regions of Ilia-Messinia, Attica, Euboea, Fokida-Lokridos and Atoloakarnanias. From 1977 to 1980 he was Director of the Archaeological Museum of Delphi. Prof. Themelis, conducted excavations in all of the above mentioned above areas. From 1980 to 1984 he served as Head of the newly established directorate for Speleology Palaio-anthropology, while in 1984 he was elected Professor of Classical Archaeology at the University of Crete in Rethymno, where he worked until his retirement in 2003. He is member of numerous scientific societies around the world (among which the Archaeological Society, the Society Euboea Studies, the German Archaeological Institute, the Austrians Archaeological Institute, of the European Society of Sciences and Arts, the Committee on Conservation of the Acropolis Monuments, the ICOM and ICOMOS) and president the Company Messinian Archaeological Studies. Professor Themelis has also received numerous honours for his work. During the period 1987-1989 he was appointed Vice-Rector of Academic Affairs of the University of Crete and chairman of the Research Committee of ELKE. From 1985 to 2003 he conducted the excavation of the University of Crete (Sector Archaeology and History of Art) in Section I of the District of Ancient Eleftherna Mylopotamos Crete while since 1986 directs the excavation of the Archaeological Society in ancient Messinia and the restoration project of the Ministry of Culture in the same site.