Hi guys! Today I'm going to show you how to obtain Mew in Pokemon Red/Blue/Yellow Versions
NOTE: If you've already beaten the game you probably won't be able to do this.
A Gameboy (Original, Color, or Advanced will do)
Pokemon Red, Blue, or Yellow
You Must Have Just Defeated Misty.
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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.
If you walk right up to the Youngster, the game will lock up, so you'll have to let him see you and walk up to you.
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.
Step 9: Tips
You use a Pokemon that knows HM Fly or Teleport
Bring at least 8 to 16 Poke Balls , Because Mew can be a difficult Pokemon to catch
Hope this Helps!
Step 11: Video
The video above is not mine but I included it for reference. And also because I had hardly any pics.