Rothmüller was born in the village of Trnjani near Slavonski Brod to a Jewish family,[1][2] who owned a store.

He was raised with five siblings (three brothers and two sisters): Branko, Avraham, Erich Cvi, Tilika and Elvira.

In London's Royal Opera House, Rothmüller made his debut in 1939 under the direction of Sir Thomas Beecham.

He appeared on British television in performances of Macbeth and La Forza del Destino.

HMV issued a few 78 rpm records on their plum label featuring Rothmüller in rôles from Così fan Tutte, Zauberflöte, Tannhäuser, Rigoletto, Tosca and Andrea Chenier.

He arranged the Sephardi folk songs, and in Zagreb in 1932, and Zurich in 1942, Rothmüller founded Omanut, society for the fostering of Jewish music.