Karas was a zither player at a wine tavern in Vienna. Hearing him play there by chance, the director of the film, Carol Reed, was so taken with the simple melodies of Karas' music that he asked him to compose for the film, even though Karas was not a composer. The result was "The Harry Lime Theme" (named for the protagonist, Harry Lime, played by Orson Welles). The movie was instantly successful and by the end of 1949, an unprecedented half a million copies of "The Harry Lime Theme" had been sold. The popularity of the score even caused a surge in zither sales.

Although Karas' life changed dramatically -- he even opened a wine bar that was hugely fashionable among cinema celebrities like Orson Welles and Gina Lollobrigida -- Karas disliked the limelight and retired.