This was one of 15 films that were released in 1961 that was considered for an Academy Award for Special Effects.
When the film was being viewed by the British Board of Film Classification, the occupants of the neighboring offices came in to complain about the noise it was making.
The concept for the film's British poster art and lobby cards was that the face of Gorgo was not revealed. The idea is based on the line of dialogue in the film from actor Martin Benson that Gorgo's name is derived from the Gorgons of Greek mythology. The sight of their faces would turn you to stone.
The film was originally going to take place in Japan. Later, the King Brothers and Eugène Lourié thought of having it take place in Paris, but that would have meant that the monster would have had to wade about 100 miles up the Seine to get to the city. Finally, the location was changed to first Ireland and then England.
At about 35:00 into the film, the only two actresses with spoken dialogue appear in it. All of the other women seen in it are extras that have no dialogue.