Karas was working as a zither player when director Carol Reed, during location scouting for the film, heard him playing in a beer garden. Reed wanted music that wasn't waltz but would be appropriate to the city of Vienna, in which the film was set, so he asked Karas if he would write and record the film's score. Karas agreed, and he wrote the theme based on a melody in a practice book. The zither had not previously been widely used in English or American music, but the theme became popular with audiences of the film soon after its premiere.
This song was originally released in the U.K., where it was 'The Harry Lime Theme.' Following its release as a single in 1950 (see 1950 in music), "The Third Man Theme" spent eleven weeks at number one on Billboard's U.S. Best Sellers in Stores chart, from April 23 to July 8. Its success led to a trend in releasing film theme music as singles. A guitar version by Guy Lombardo (recorded December 9, 1949, released by Decca under catalog number 24839) also sold strongly, and four other versions charted in the U.S. during 1950. According to Faber and Faber, the different versions of the theme have collectively sold an estimated forty million copies.