In Frankfurt he studied philosophy under Theodor W. Adorno who later helped him to avoid a threatening deportation (as an "undesirable asylum seeker").
In 1972 he was appointed the director of the Deutsches Schauspielhaus (German Play House) in Hamburg and stayed there until 1979.
It has given the German public the opportunity to see prominent theater productions like those of Peter Brook and Ariane Mnouchkine.
From 1985 to 1988 Nagel was the director of the Staatstheater Stuttgart, and from 1989 to 1996 Professor of History and Aesthetics at Berlin University of the Arts.
His book about Mozart's Opera Autonomy and Mercy has been translated into English, French and Japanese.