Waronker began to establish his music career in Germany, but was forced to return to the U.S. when Nazi Party members began to threaten Jews. From 1939 to 1955, Waronker recorded music for 20th Century Fox films. He then chose to enter the music industry independently, co-founding Liberty Records and serving as its chairman. The first artist he signed was popular singer Julie London.
One of the major acts who recorded on Liberty Records was The Chipmunks, actually Ross Bagdasarian, who recorded as David Seville. Seville named The Chipmunks (Alvin, Simon, and Theodore) after Liberty Records executives Alvin Bennett, Simon Waronker, and Theodore Keep, respectively.