Jet Jaguar was the earliest example of a fan-service in a Godzilla film, the result of a contest Toho had in mid-to-late 1972 for children to come up with a new hero for them to use (to capitalize on the many tokusatsu and anime superhero and super robot shows that were all the rage at the time). The winner of the contest was an elementary school student, who submitted the drawing of a robot called Red Arone, which superficially resembled both Ultraman and TranZor Z (both of which were very popular at the time). The robot was renamed "Jet Jaguar" and was set to star in a film vehicle for him, titled "Jet Jaguar vs. Megalon," which pitted him against Megalon (a previously unused Godzilla enemy design). However, Toho figured Jet Jaguar would not be able to carry the film on his own, either in screen appearance or marketing value (which was very important to Toho), so they shut the project down during pre-production after doing some tests and storyboards. Less than a month later, screenwriter Shinichi Sekizawa was called in to rewrite the script to add Godzilla and Gigan, providing more marquee value (especially since Godzilla was still very popular with children). After revised pre-production, shooting took a hasty three weeks to make up for lost production time. Due to this delay, the total production time, from planning to finish, was six months.
This film was originally produced in 1973 but was not released in the United States until 1976. A small US distributor, Cinema Shares, hoped to ride on the popularity of 'Dino De Laurentiis' 's highly promoted production King Kong. This became especially apparent in the US poster art, which depicted Godzilla and Megalon battling each other on top of the World Trade Center towers.
According to Teruyoshi Nakano, the Godzilla suit used in this film (nicknamed the "Megaro-Goji" suit) was made in a week, making it the fastest Godzilla suit ever made to date. They didn't have time to make the eyes work correctly, something they had more time to fix for Godzilla's appearances on Toho's superhero TV series Ryusei Ningen Zon, which was produced around the same time.
Aside from playing the Seatopian Emperor Antonio, Robert Dunham, who was also a stuntman and vehicle racer, also played one of the two the "Unit 1" Seatopian Agents (he was the one on the motorcycle) in one of the car chase scenes (where he and another agent were chasing Yutaka Hayashi's character). The last shot where he gets splashed on with cement was done with another actor, because Dunham had to get to the studio that day to record his part, and didn't want to have to get cleaned up on the way.
Megalon was originally created to be the third evil monster (besides King Ghidorah and Gigan) in Godzilla vs. Gigan (in its second draft, "Godzilla Vs. the Space Monsters: The Earth Defense Command"), but budgetary cuts nixed Megalon from that film altogether. While he was originally created to be an opponent for Godzilla, he was considered for the role of the lone villain of this film when it was planned as a Jet Jaguar solo vehicle ("Jet Jaguar Vs. Underground Monster Megalon"), before, ironically, Godzilla was written into the story along with Gigan.