Along with Pokemon's Official Mew merhcandise and the fanbase's dedication to Pokemon 151, it has made many appearances in the anime series and movies.

Anime

There has been a few episodes with Mew in it. They are mostly just cameo appearances though, Mew being the rare Pokemon it is.

The Lean, Mean, Rocket Machine

In the anime, Mew appears in a daydream of the boss for Team Rocket, the antagonists of the Kanto franchise. In the dream, the boss is surrounded by many rare, ancient, or legendary Pokemon, Mew being one of them to the right next to Milotic and Riolu.

Master of the Mirage Pokemon

Another Mew appeared in Master of the Mirage Pokemon. While this Mew was close to a hologram and not a physical Mew, it's a Mew nonetheless. In the episode, this ghostly white Mew, unlike the other Mirages, possessed a soul, which angered it's creator. The Mew then perished at the end after fighting against a mirage Mewtwo, saving the protagonists of the series.

File 4: Chrizard

The last true physical Mew that appeared was the Mew in the ending of Pokemon Origins Season 1, or File 4: Charizard. After Red, the protagonist, caught Mewtwo, he states we wishes to catch Mew next, which is when the Mythical Pokemon flies by the nearby window, pauses, and chuckles before flying off screen.

Minor Appearances

Mew appears on Alexa's Magazine.

Mew appeared on a computer screen used by Mewtwo's Creators in Genesect and the Legend Awakened while Mewtwo described it's past with humans in relation to the Genesect clan.

A Mew made a small appearance in Meloetta's Moonlight Serenade.

Movies

Mew's first appearence was in Pokemon the First Movie. It played the role of defeating Pokemon Mewtwo had cloned from the originals, Mew leading the originals in the revolt. They suceeded, and Mew is seen to fly back to a mountain seen at the start of the movie, leading veiwers to believe the mountain is Mew's home.

In this movie, Mew guards the mysterious tree of Life, which flows aura into the Earth and allows it to live. It soon grows sick after releasing the protaginists from the tree (the tree thought they were a virus). After Mew grows sick, Lucario sarafices itself to save the Pokemon, Lucario vanishing and Mew living on to keep the planet alive.

Mew also appeared in the opening to Destiny Deoxys.

 

 

Lastly, while we do not know if Mew will appear in Pokemon Sun and Moon, below is a countdown to the Game's release.

Let's hope for Mew in this new game!