Peter Cushing did double duty on this film. In additional to his own role, he would feed Louis Hayward the lines for the split screen shots. Director 'James Whale' initially cast him only to play opposite Hayward in the sequences where both twins appear together, but was impressed enough with the newcomer that he offered Cushing a small part on horseback. This was Peter Cushing's film debut, and he had the unique opportunity to view his own rushes and improve his own performance, especially since none of it would be used in the finished feature. As 'Second Officer,' he can be seen 17 minutes in, with two lines of dialogue ("I've been here before").

Nearly all of the characters in this film actually existed, but none of the characters who die in it actually died that way in real life.

In 1952 'Alan Hale Jr.' played Porthos Jr. in a quasi-sequel, At Sword's Point (1952) and recreated his father's role in the remake The Fifth Musketeer (1979).

In 'Alexandre Dumas pere''s novel, The Vicomte de Bragelonne, Philip is substituted for Louis XIV for only a short time before regaining the throne and re-imprisoning his brother. In this film, Philip is successfully made a permanent substitute. Subsequent adaptations of the book have more closely followed this film than the original novel, sometimes even crediting this film's screenplay as their basis as well.

Nigel De Brulier previously played Cardinal Richelieu in The Three Musketeers (1921), The Iron Mask (1929) and The Three Musketeers (1935).

James Whale previously directed Frankenstein (1931) and Bride of Frankenstein (1935) while Peter Cushing (Second Officer) later played Baron Frankenstein in The Curse of Frankenstein (1957), The Revenge of Frankenstein (1958), The Evil of Frankenstein (1964), Frankenstein Created Woman (1967), Frankenstein Must Be Destroyed (1969) and Frankenstein and the Monster from Hell (1974).

Fouquet's statement that "we must all hang together, or we shall assuredly hang separately" is thought to have been spoken by Ben Franklin at the signing of the Declatation of Independence.

Heavy metal band Iron Maiden got their name from this particular adaptation of the classic story. Band founder Steve Harris stated that he had seen it on TV one night in 1975. After having seen the picture, it made him think of the infamous torture device, thus giving the band it's now world famous moniker.

Included among the American Film Institute's 2001 list of 400 movies nominated for the top 100 Most Heart-Pounding American Movies.

The "iron mask" does not make its first appearance until well over an hour into the film.