This isnt really a cheat, but a glitch in the game. You may need to start over to get Mew, but it is worth it.First you need fly, the third gym badge, a Pokemon lv 10ish and a Pokemon with sleep powder. North of cerulian city is a trainer with a lv 17 ish slowpoke, MAKE SURE YOU DO NOT FIGHT HIM, if you do, you can not get Mew also there is a gambler with some water Pokemon left of lavender town, don't fight him either.
Okay, get a flying Pokemon, a Pokemon with an attack with sleep powder or spore, a level 10 ish Pokemon like rattata, and a bunch of pokeballs, preferably great balls or ultra balls. Teach the flying Pokemon fly, then go to the first town you go into in the game with a Pokemon center, talk to the guy who shows you how to catch Pokemon and let him show you. Then fly to lavender town.save your game, then go west of lavender town, avoid the gambler and go right in front of the enrtence to the underground path. Go south and immidiately press the start button then fly to cerulian city, the gambler will see you as you leave. When you arrive in the town, your buttons will not work, don't be alarmed. Simply go north and let the slowpoke trainer see you so that it walks up to you, either that or talk to him, if your game freezes, restart and do the gambler thing a gain then try the other slowpoke trainer method. Defeat him and your buttons will work again. Fly back to lavender town, then go east and save your game at the entrance of the path to the route, when you enter the route, your menu will open, push b to close it (you have to push b or it wont work) When you close the menu, you will hear the traditional music and scene before a battle and after the scene is over you will see a Mew on the screen. Battle it with the lv 10 ish Pokemon you brought and when it is low enough on health, switch to you sleep powder/spore Pokemon and put it to sleep, then chuck great/ultra balls at it untill you catch it, now you have a mew.
This is tested and works, I don't know if it will work for yellow, but I have used it more than enough times for it to be believable. You don't need a game shark or anything, and it shouldn't ruin your game,
P.S. This was written from memory so I'm not sure if it accurate, but I did the glitch from memory, so it should be fine
Added by: Aj0_o Jul 23rd 2008, ID#4917