 From 1964 until 1971, he was assistant at the Institute of Linguistics at the University of Würzburg, where in 1969, he passed his habilitation for Comparative (Indo-European) linguistics.From December 1971 onwards, Mittelberger was professor for Comparative (Indo-European) linguistics at the University of Graz, and from 1973-1988 also director of the Institute for translation studies ("Institut für Übersetzer- und Dolmetscherausbildung").His research centered on the proto-Indo-European language, the Indo-European languages of (old) Anatolia, Indo-Aryan languages (Sanskrit, Middle Indic), Iranian (Old Persian, Avestic, Middle Iranian), Greek (including Linear B), Latin and languages of old Italy, as well as Germanic with special interest in Gothic.