Hammer Film Productions was originally due to film the novel "I Am Legend" by Richard Matheson under the title "Night Creatures," but abandoned the project when the BBFC informed Hammer they would not pass the film. With a film titled "Night Creatures" committed to Universal-International, Hammer Film Productions re-titled the already completed "Captain Clegg" to fill the obligation.
The film was based on Russell Thorndike's "Doctor Syn - A Tale of Romney Marsh," but Dr. Syn's name was changed to Blyss to avoid legal problems with Walt Disney, who claimed exclusive rights to Thorndike's stories.
Oliver Reed and Yvonne Romain had recently appeared - in separate scenes - in The Curse of the Werewolf (1961).
Despite the story being set in 1792, the hymn "Oft in danger, oft in woe," sung by the church choir, wasn't published until after the author's death in 1806.
When the sailors march out of the churchyard on their way to their first raid, they pass a red-brick house with lobed gables on their right where Parson Blyss lives. This is Hills House in Denham, once home to Alexander Korda and Merle Oberon, and then to Sir John Mills whose daughter Hayley restored Cobstone Windmill, which is also featured.