Ami was not included in the original proposal for a hypothetical Codename: Sailor V anime, which instead featured Minako's very similar-looking best friend from that series, Hikaru Sorano. She was present, however, by the time the concept was expanded to center on Sailor Moon.
Creator Naoko Takeuchi designed Ami as the "team brain," giving her genius-level intelligence to create the impression that she was not quite human—in fact, the character was originally intended to be a cyborg with an accelerator. One possible storyline involved her losing an arm or being injured in some other way and dying from it, but Takeuchi's editor objected, so Ami became a fully human character.
Sailor Mercury's original costume design, like the others', was fully unique. It featured full-length sleeves, pink ribbons, shoulder guards, green accents, buttons on the stomach, and high-tech goggles. Later, Takeuchi was surprised by these sketches and stated that she did not remember drawing them. She also describes Ami as looking like Noriko Sakai, a J-pop idol of the early 1990s, and in Ami's original debut, Usagi thinks to herself that Ami resembles Miss Rain, a character from another Takeuchi series. This reference was removed in the 2003 renewal manga. In the English manga, a similar comment was made, but referencing Demi Moore.
Certain background details of Ami's character were chosen symbolically—for instance, her Western astrological sign is given as Virgo, which in astrology corresponds to the planet Mercury. In reference to a popular Japanese belief, her blood type is given as A, supposedly indicating sensibleness and fastidiousness.
The kanji in Ami's last name translate as "water" (水 mizu) and "field" or "civilian" (野 no); and her first name translates as "Asia" or "second" (亜 a) and "beauty" (美 mi). It is structured as a pun, as the syllable "no" indicates a possessive, so that her name can also be understood as "Ami of Water." It is frequently mistranslated as "Friend of Water" because of the French word ami, which is included in some Japanese dictionaries.
In the English manga and other books, "Amy" retains her family name of Mizuno. One early episode of the anime shows her answering the phone in her house by saying "Anderson residence," where in the original she had answered with "This is Mizuno," but this surname never appears again, and she is reverted to Mizuno by Cloverway in the series' third season.