The film was originally going to be set in Chicago, with the rocket crashing in Lake Michigan. Right before submitting the idea to producer Charles H. Schneer, Ray Harryhausen decided to change the setting to Italy at the last minute, after deciding that he always wanted to go on vacation there.
Since he planned to use a real elephant for some of the footage in the zoo, Ray Harryhausen asked for one that was 15 feet tall, but the person responsible was only able to procure an eight-foot-tall one for him. In order to make the elephant look much bigger, a 4'6" actor was cast to play the zookeeper.
The Ymir roars in the film are variations of elephant roars sped up and modulated in pitches at different rates.
Upon completion of this film, Ray Harryhausen felt that, after doing destructive modern-day sci-fi monster thrillers, his interests had shifted into doing fantasy adventures (with monsters) set in a romantic past, a trait that began with his next film, The 7th Voyage of Sinbad (1958). Thus, "20 Million Miles to Earth" was his last film to have a purely modern-day setting. (Only one other subsequent film of his, First Men in the Moon (1964), has bookends set in the present.) This was also his last film in black & white.
Ray Harryhausen's original design for the monster was a giant cyclops, similar to the one he later used in The 7th Voyage of Sinbad (1958). He discarded the idea after making a clay model of it, and eventually settled on the reptilian Ymir.
Ray Harryhausen: a man feeding peanuts to the elephant that later battles the Ymir. He did so because the actor scheduled to play the part didn't show up. He later appears in a crowd fleeing the zoo.