Unlike other characters in the Pokémon
franchise, Mew's development was not overseen by Ken Sugimori
, but by Game Freak programmer Shigeki Morimoto. Morimoto programmed Mew into the game secretly, as a prank amongst the staff just prior to its release in Japan, intending it to be a Pokémon only Game Freak staff members would know about and be able to obtain.
Mew was added at the very end of the development of Pokémon Red and Green
after the removal of debug
features, freeing up just enough space to add the character despite being told not to alter the game any further at this point. Though not intended by the developers to be obtainable, due to a glitch, players were able to encounter it.
In the spring of 1996, Game Freak president Satoshi Tajiri used the Japanese manga journal CoroCoro Comic
as an experimental exhibition of Mew and distributed the first cards of it for the card game as free giveaways,
which surprised many at Game Freak, including Morimoto.
Due to the success of the experiment on April 15, 1996, Game Freak announced a contest to publicly release Mew to 151 winners.
Tajiri described using Mew to create hype around an "invisible character" within the game and to keep interest alive in the title and create rumors and myths about the game passed around by word of mouth,
which resulted in increased sales for the game.