How to Find Mew in Pokémon Red/Blue

Gambler and Youngster Glitch

Restart your saved game if you went too far in the game. This glitch requires you to battle Gambler and Youngster.

Go in front of the Gambler below the underground path from Lavender Town to Celadon City and press the Start button immediately. Instead of not letting you pause the game, your Pause Menu will appear.

Fly to Cerulean City. The gambler will see you after you press fly and you'll hear his music, but you will then fly to Cerulean City anyway. Your start button won't yet work, but it will shortly after.

Go up to the Youngster in the bush area just after Nugget Bridge. He's the second Youngster you come across and he's facing up. He's also standing above a "Lass". He's very easy to beat now, as he only has a Slowpoke.

Defeat the trainer and fly all the way back to Lavender Town. Your start button should work.

Walk to the left exit of the town. Your pause menu should pop-up automatically.

Exit out of the pause menu to begin battle. Be careful because Mew will only be at level 7!

Catch the Mew with the Master Ball or weaken it using attacks. Or, if it is weakened enough, try catching it with a Poké Ball.

Trainer and Youngster Glitch

Restart the game if you went too far. This glitch requires you to battle the trainer hidden in the grass and Youngster (both in Cerulean).

Catch an Abra (if you don't have another Pokémon that knows Teleport). You can find it on Routes 24 & 25 in Pokémon Blue/Red and on Route 5 (you may need a Pokémon who can put it to sleep).

Go to the north of Cerulean and stand in the same spot of the character in the photo/video.

Save.

Walk forward. Press start at the same moment.

Select Abra and Teleport. The trainer will see you but you'll find yourself at the Cerulean Pokémon Center anyway.

Go to the Youngster (route that takes you to Bill); he's the third one. Make sure you don't get into any random battles before him and to leave a gap between him and you (so he'll be able to walk).

Beat the trainer (he has only a Slowpoke) and use Teleport again. You'll be outside the Pokémon Center again.

Walk north again. At some point the Menu will pop up. Exit it and you should find yourself in a battle with Mew!

Using a Glitch to Catch any Pokémon

Extend your catching prowess. Now that you know how the trick works, use it to catch other Pokémon. Fighting the Youngster will always result in a Mew encounter because his Slowpoke has a Special of 21 (15 in hex). But if you fight something else before returning to Route 8, it will change which Pokémon you encounter. For example, say you chose a starting Pokémon other than Bulbasaur and regret your decision. Bulbasaur's hex number is 99, which converts to 153 in decimal. Now, you could fight a wild Pokémon with a Special of 153, but the only place to find that is on a Level 64 Chansey in the Unknown Dungeon, and those Chansey are quite rare. Worse, they only have a one-in-sixteen chance of having the right Special. Fortunately, there is a solution: Ditto's Transform, which copies the stats of the opponent. This means that you can catch any Pokémon in the game by following the steps set forth in this section.

Catch or raise a Pokémon with the proper Special stat (153 for Bulbasaur). You can find a complete list of which decimal value is needed for each Pokémon at the bottom of the page.

There. Fly away from the Gambler battle.

Fight any trainer that can see you from a distance and walk up to you (this is necessary to both unlock your Start button and trigger the Route 8 battle).

Find a wild Ditto and let it Transform into your Pokémon with the proper Special.

Beat the Ditto (or just run away) and immediately fly to Lavender Town without fighting anything else.

Enter Route 8, and Bulbasaur (or whatever) will appear.

Note that since only the least significant of the two Special bytes is used for this, you can also add 256 to the values listed above and use that Special to get the same Pokémon. This is important because there are a few Pokémon, such as Kangaskhan, whose identifying value is lower than the minimum Special that any Pokémon can have. In Kangaskhan's case, you can find her by using a Special of 258 (2 + 256). Also note that when you perform this trick, any trainers you fight after the Gambler will be "used up." Make sure you have enough unused trainers to catch all the Pokémon you want! Fortunately, the Gambler can be used to initiate the trick an unlimited number of times.

Gambler and Biker glitch

Restart your game if you went too far. This method requires you to battle Gambler and Biker.

Go in front of the Gambler below the underground path form Lavender to Celadon and immediately press start.

Fly to Fuchsia City (after that, your start button will temporarily be disabled).

Walk to Bicycle Road and then go up.

Once the bike is moving up slowly, keep going up. When you see water in front of you, turn right.

After that, go up until you see grass. The Biker you'll need to battle will be facing the grass.

Go up to the Biker. Battle him.

After you battle him, your start button will work again. Fly to Lavender Town, and then go left towards route 8. You're start button will then pop up automatically. Press out of it.

After that, there's your Mew. Catch Mew with a Master Ball, or weaken it, and then catch it with a Pokeball.