A friendship with composer Richard Harvey, which grew from their shared interest in ethnic music and instruments, led Gregson-Williams into film music. Gregson-Williams had been intrigued by the use of classical music in film since Visconti used Mahler's music in Death in Venice, and was happy to assist Harvey after the latter got some work scoring for British television projects. The two of them worked together in collaboration with Elvis Costello in GBH and Jake's Progress, two films directed by Alan Bleasdale. In the process, Gregson-Williams learned much about advanced orchestration. Gregson-Williams's first original score was for an independent film by Genevieve Jolliffe, called Urban Ghost Story, which was a coming of age drama rather than a horror picture. Gregson-Williams composed an intimate score, mainly for solo piano and flutes, but of a substantial length, 50 minutes. Following that, the established film composer Hans Zimmer asked Gregson-Williams to join his organization Media Ventures, which provides music for film, television, advertisements, and other commercial uses, including the two-minute piece Gregson-Williams wrote for the World Cup soccer finals in Japan. Gregson-Williams worked directly with Zimmer on the score to The Prince of Egypt, composing certain sequences in the film using Zimmer's melodies to the composer's specifications. Other film and television projects on which Gregson-Williams has worked include El Dorado, Virtually Sexuality, Muppets From Space, Extremely Dangerous, and Hannibal.
Brother Harry works for Media Ventures' Los Angeles operation, while Rupert mostly works from his fifteenth-century Sussex countryside home. The two brothers collaborated on the score for Jerry Bruckheimer's film Swing Vote. Rupert uses Macintosh MIDI equipment and is fond of Steinberg's Cubase sequencing software, which allows him to produce an accurate synthesized version of a cue, to play for a director before orchestrating and recording it.