Fehenberger was born in Oberweidach, Upper Bavaria, and began singing as a boy in a church choir, later studying voice with Elisabeth Wolff in Munich.
He made his stage debut in Graz, as the Italian singer in Der Rosenkavalier, in 1939.
He then sang at the Staatsoper Dresden from 1941 to 1945, and made his debut at the Munich State Opera in 1946.
He sang regularly at the Salzburg Festival between 1949 and 1962, where he created the role of Haemon in Carl Orff's Antigone.
Fehenberger was much admired as Lohengrin and Walther, but also gained considerable acclaim in Italian roles such as Duke of Mantua, Riccardo, Alvaro, Radames, Pinkerton, Cavaradossi, as well as Bizet's Don Jose.