Studied at the Vienna Conservatory (today MUK) and at the Vienna University of Music (today MDW) and completed his studies (artistic and educational) with the Magisterium der Künste. He was a member of the Gustav Mahler Youth Orchestra and the Vienna Jeunesse Orchestra. As a substitute he has worked in the orchestra of the Vienna State Opera, in the orchestra of the Volksoper Vienna, in the ORF Radio Symphony Orchestra Vienna and in the Tonkünstler Orchestra Lower Austria. He is a member of Ensemble XX. Century. He also played in the Klangforum Wien, in the Ensemble PHACE, in the Concilium musicum Wien and in various other ensembles for new and old music. Various cross-genre projects and multimedia concerts (e.g. "Light Painting", "Across Africa") expand his artistic radius.
Harry Demmer is the co-founder of the "Wiener Mallet- und Percussionensembles" und 'DrumArtic"; he is the artistic leader of the MUKpercUssionWIEN. „Wiener Mallet- und Percussionensembles“ und „DrumArtic“; er ist der künstlerische Leiter von MUKpercUssionWIEN.
Harry Demmer has been a lecturer at the Music and Art (MUK) University of the City of Vienna since 1991. From 2017 to 2018 he was a lecturer at the University of Music and Performing Arts Vienna.