During this period, he participated in excavations at Thessaloniki, Pella, Lefkandi on Euboea, Stratoni in the Chalcidice, and the palace at Vergina.
In 1972, Themelis received a doctorate from the Ludwig Maximilian University of Munich for his thesis on Frühgriechische Grabbauten (Early Greek funerary structures).
In this role, he published several books and articles, including studies of the works of the sculptor Damophon of Messene that were found in the excavations.
He was chairman of the "Society for Messenian Studies" and a corresponding member of the German and Austrian Archaeological Institutes [de].
On 18 January 2005, Themelis was appointed a commander of the Greek Order of the Phoenix by president Konstantinos Stephanopoulos.