This was one of 15 films that were released in 1961 that was considered for an Academy Award for Special Effects.
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.
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 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.
Aside from Japan, Paris, Ireland and finally England, the producers were considering using Australia as a location; however, they decided that no one would even care if Gorgo's mother attacked Australia and thus settled on England.