[citation needed] Between 1921 and 1928, Demus studied art history at the University of Vienna under Josef Strzygowski, receiving his Ph.D. summa cum laude.
[citation needed] In 1946 Demus returned to Austria, accepting a position as president of the newly organized Bundesdenkmalamt (Federal Monuments Office), a post he would occupy for nearly twenty years.
Late in life, already having turned 70, Demus embarked on the most ambitious scholarly project of his career, namely the restoration and documentation of the mosaics of San Marco in Venice.
Having now turned 80, Demus undertook one final project, the documentation of the late medieval art of Carinthia, where he had served in the monuments service in his youth.
Demus was the consummate Viennese art historian, trusting his eye and developing a massive visual repertoire through the painstaking first-hand inspection of monuments.