Peter Sellers made this film in part as a means of emulating his hero, Alec Guinness, by playing multiple roles in one movie.
Among the musical quotations used by Edwin Astley in the film were excerpts from Felix Mendelssohn's "Hebrides Overture", "Rule Britannia", "A Life on the Ocean Waves", "Frankie and Johnnie", a number of American marches, including a number of passages from "Our Director" and Astley's own score for The Adventures of Robin Hood.
The AFI online Catalog reports the showing of this film, "The Mouse That Roared (1959)," to diplomats in Geneva, Switzerland on 23 May 1959. There was no mention of the circumstances of the showing.
The Marseilles and New York harbor sequences were filmed in Southampton, UK. The presence of the Queen Elizabeth ocean liner there was a lucky coincidence.
The New York invasion sequence was filmed in Manhattan on a Sunday morning, accounting for the city's empty streets.