Mimey debuted in It's Mr. Mime Time. Ash and his friends first saw Mimey using its Psychic powers to climb an invisible barrier that 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 was officially captured by Ash sometime offscreen after this.
In Z-Move Showdown!, the closing ceremony of the Manalo Conference was disrupted by the appearance of a Guzzlord. Mimey accompanied Delia and Professor Oak as they tried to leave, only for a ShinyGuzzlord to block their way. However, it was quickly seen off by Professor Kukui and Ash.
In Legend? Go! Friends? Go!, Mimey traveled with Delia, Ash, and Professor Oak to Vermilion City to attend the opening of the Cerise Laboratory. After Ash and his new friend Goh encountered a Lugia and reported this to Professor Cerise, Cerise decided to make Ash and Goh his new research assistants and gave them a room at the lab. Delia then decided to leave Mimey at the lab so it could watch over Ash.
In Betrayed, Bothered, and Beleaguered!, Mimey was reunited with Delia when she came by to visit Ash. However, as Delia left, Mimey learned that Pikachu, who had been feeling neglected by Ash lately, was planning on leaving with her and tried to stop him. When Pikachu refused, Mimey decided to accompany him to Pallet Town. During their journey, Mimey was attacked by a group of Spearow, but Pikachu saved it by using Thunderbolt. By the time they arrived in Pallet Town, Pikachu had changed his mind and turned back to return to Ash, only to find out that Ash had come to Pallet Town looking for him.
At first, Mimey only listened to Delia and if anyone else gave it orders or asked it to do something, it normally ignored 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 is 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. As shown in Ivysaur's Mysterious Tower!, Mimey can perform tasks such as cleaning and vacuuming by pantomiming. It was later shown "eating" with Ash and Goh by pretending to peel and eat a banana.
Ash was 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.
In Pokémon Journeys: The Series, Ash was revealed to be a good partner for Mimey in Pokémon battles as well. In Serving Up the Flute Cup!, Ash was able to take advantage of Mimey's unusual flexibility and defensive capabilities to withstand the assault of Hodge's Hariyama in the finals of the Battle Frontier Flute Cup. However, Mimey is not as reliable as some of Ash's regular Pokémon, as shown as when it immediately called it quits after their first victory as a team. Mimey is also a good sparring partner for Ash's Riolu, being able to take multiple Vacuum Waves from the newborn Pokémon while its partner focused on improving its technique. In addition, Mimey tends to be the voice of reason whenever there is conflict.
It is also the first Pokémon he has owned to be given a nickname. However, Mimey is only referred to by its nickname by Delia, while other characters refer to Mimey by its species name. Ash does use the nickname in the English dub, however.
Mimey is the first recurring Pokémon in the anime to have had its type change confirmed.
Out of all of Ash's non-temporary Pokémon, Mimey has gone the longest without being used in battle by Ash after its capture, at 1,028 episodes.
While it is unknown how exactly Mimey came into the ownership of Delia, some Japanese supplementary material list it among the Pokémon Ash has caught. Whether this means that Mimey is a Pokémon Ash caught and gave to his mother, or a Pokémon belonging to him that he keeps at home is unknown.
Mimey is the only one of Ash's Pokémon currently still in his possession to have not posed and appeared in the photo of all of Ash's Pokémon up to that point in the final episode of Pokémon the Series: Black & White.