Mimey debuted in It's Mr. Mime Time. Ash and his friends first saw Mimey using its Psychic powers to climb an invisible barrier it created. Ash wanted to catch it, but a ringmaster named Stella wanted to catch it as well to inspire another Mr. Mime that worked on her circus. In the chaos, Mimey got away. Brock decided to help and had Ash dress as a Mr. Mime to help instead. When Delia and Professor Oak went to the circus, they found out about the plan. When the wild Mr. Mime Ash and his friends saw earlier came to her door, Delia thought it was Ash in costume, and provided it with food. When the real Ash showed up, still in costume, she was quite puzzled. It then teamed up with Delia to help stop Team Rocket from stealing all the Pokémon in Stella's circus. Ever since then Delia has adopted it and affectionately nicknamed it "Mimey". It is unknown if Delia actually caught Mimey, as she is never shown with its Poké Ball.
Mimey will only listen to Delia and if anyone else gives it orders or asks it to do something, it will normally ignore them unless told otherwise by Delia. It greatly enjoys doing the household chores around the Ketchum home and does not like sharing its job with others. When Ash, Misty, and Tracey returned to Pallet Town following their journey through the Orange Archipelago, it was revealed that Brock and Mimey had become rather competitive regarding household tasks while the two of them were sharing the house with Delia. It's also protective of Delia's property and will quickly spring into action to defend it as seen in The Rivalry Revival where it used Light Screen to keep Team Rocket from trampling her garden.
Ash was shown to be somewhat envious of Delia's affection for Mimey; Delia once described it as more polite and helpful compared to Ash. He also became annoyed when Delia took its side after it had woken him up by using the vacuum on his face. Despite this, Ash and Mimey are on good terms, with Mimey happily accepting Ash's friendly greetings on his return trips to Pallet Town. Ash in particular is glad that Delia has Mimey as a housemate, as while he is on his journey, his mother will have company at home and will never be lonely.