He began his career as a pianist but made his debut as a conductor in 1894 at the Cologne Opera. By 1900 he was at the State Opera in Berlin, and in the following year he became Gustav Mahler's (Mahler, Gustav) associate in Vienna—the beginning of what was to be a lifetime spent in promotion of the master's music; he conducted the premieres of Das Lied von der Erde (1911) and the Ninth Symphony (1912). Walter moved to the Munich Opera (1914–22) and from 1922 conducted at Salzburg, where his interest in Mozart developed. Other appointments followed: at the Berlin Municipal Opera (1925–29) and as Furtwängler's successor in Leipzig with the Gewandhaus Orchestra (1929–33). The advent of the Nazi regime in Germany forced him to leave Leipzig and his Berlin concerts; he moved first to Vienna (1936–38), then to Paris, and finally to the United States (1939). He conducted frequently at the Metropolitan Opera and the New York Philharmonic (musical adviser, 1947–49).