Born in Miroslav (now in the Czech Republic), he taught in Jaffa, Israel, at the Kiryat Sefer agricultural school from 1907 to 1909 and from 1911 worked in the Jewish communal library in Vienna, Austria. When the Nazis closed the library in 1938, he migrated to England. His main scholarly was on Jewish mysticism including a book on the Zohar, a translation of sections of the Zohar into German and History of Jewish Mysticism, which appeared in London in 1946 (translated from the German by Maurice Simon). Mueller translated into German works by Abraham Ibn Ezra and Bialik and prepared a stage version of Plato’s Symposium.