Mew was never made available to catch normally in the game, but there are a few different glitches that can be used to catch a Level 7 Mew. I tested one of them and it worked perfectly, so I'll post it for those who don't know. I think this version of the glitch works for all 3 Gen I games, but I only tested it in Yellow. I advise reading through the whole explanation before attempting it, so that you're ready.

After the first time you finish Nugget Bridge, there are two trainers you must avoid for the glitch to work. The first one is the trainer directly to the left of the bridge in the grass. The second one is a trainer along the route that leads to Bill's House. This trainer uses a Slowpoke and he faces towards the top of the screen (see picture below).



There are four trainers, counting the player. The guy on the far left and the girl to the right are both unimportant. The boy that is facing upwards who is also higher on the screen than the other three must NOT be challenged. You will need him later for the glitch to work. The player must go the direction that he is facing in the picture in order to avoid the trainer.

Next part. When you have finished visiting Bill and have access to Route 5 (the route below Cerulean City where the Daycare is), you can catch an Abra. The glitch requires that you have a Pokémon that can use either Fly or Teleport. When you have an Abra, go to Cerulean City and heal your Pokémon so that this will be set as Abra's Teleport location. This Teleport destination is also required.

Before you start, make sure you have plenty of Pokéballs for Mew and put the Pokémon you plan to use on the second trainer's Slowpoke in the lead (I advise Pikachu if it knows Thundershock and is strong enough). You will not be able to switch Pokémon in that battle or the glitch will fail and pretty much stop your game. I think the glitch may also fail if your Pokémon faints. It is also important that you save before you start the glitch and do not save until after you have caught Mew. If you make a mistake in the glitch and save, you will unable to restore your game. Also, you could run out of Pokéballs while you try to catch Mew or accidentally cause Mew to faint. If you mess up the glitch, it is impossible to retry it unless you can return to a save from before you started. The next image shows how to start the glitch.



This location can be reached by turning left at the end of Nugget Bridge. Move the player down one space. Ordinarily, this would trigger a battle with the trainer. In this case, you need to pause the game, by pressing Start, before the battle can initiate. I used button mashing while the player sprite was moving. That worked for me, but you could break your keyboard/controller if you're not gentle.

While in the Menu, go to the list of Pokémon, select Abra, and use Teleport to return to the entrance of the Cerulean PokéCenter. The exclamation point will appear above the trainer's head right before the Teleport animation starts. After the Teleport, you will be unable to open the Menu. This means you did it right so far. Next head out to the second trainer. See the next image below.



It is important that you walk into the trainer's view from this distance (along the wall). If you do it from any other angle, the glitch will fail and the game may crash. The trainer will notice you and a battle will start. He just has a Level 17 Slowpoke. During the battle you must not do anything other than use Fight. If you try to use PKMN, ITEM, or RUN, it will prevent you from continuing (it happened to me). Just stick with beating the Slowpoke as fast as you can.

When the battle ends, the Start Menu should work again. Use Teleport again to return to the Cerulean City PokéCenter. You're almost there. Head for Nugget Bridge again. When you enter the area (the background color changes), the Menu will open automatically without you doing anything. Press B and a wild Pokémon battle will begin. A Level 7 Mew will appear. All you need to do now is catch it. You can enter the Item bag or the Pokémon menu for this battle. You can weaken it, but make sure it doesn't faint. If you brought Pikachu and it knows Thunder Wave, I strongly advise using it to paralyze Mew. This will make it easier. I also weakened it using Tackle with a Magikarp that I bought.
When you have successfully captured the Mew, you can continue the game normally. Here's my victory picture.



That's all for this guide. If you would like a better visual aid, you can check this YouTube video that I followed:

http://www.youtube.com/watch?v=novtRYNk06c

*This is tagged to the game, so it will appear on that page as a reference source for other players. I used Bulbapedia and the included YouTube video to build this guide. These are the steps I followed that led to a successful capture. If anyone thinks I missed something let me know.

This is a pretty thorough article. Although I already knew all of this, this is a pretty detailed way of explaining it. Thanks.

You must've been bored, eh?

Nice way of explaining it for all the newbs who wantz teh legendaryzorz Pokemawnz :3

awesome. i hope this works on the emulator.

vizwiz123 :
I was kinda bored, but I never knew about the Mew Glitch until I found it online. I just figured some other people might not know about it and would like to train a Mew. I still won't use the glitch on my Red cartridge because I'm afraid it could trash my Stadium cartridge (I still don't have a Gameboy system so I play it through the GB Tower).

thepvtcaboose : It works! How do you think I have my Mew on vizzed's pokemon yellow
(Note: I haven't trained Mew yet, so it has low stats D: )
Btw I use the java emulator. Anyway, here is the proof:

I knew how to get mew and also have all 151 pokemon without trading. I use the mew trick and ditto trick to get all of them and got my diploma!






Image upload: 500x300 totaling 27 KB's.

noelia : Your a bamf Noelia! Thats so dope! Now i gotta do this.

thepvtcaboose : lol, actually it's very easy, if you follow the instructions.
Anyway, good luck with Mew's hunt

Lmao, how do people figure this stuff out? Still thanks

ewag1997 :
I really have no idea how someone could have figured this out. Some people just spend their time looking for this stuff and trying to find how to exploit it in a way that will benefit the player. If you can do something that you shouldn't be able to do in a game, it could lead to something that shouldn't happen. That applies to a lot of glitches.

It is possible to run into Mew a second time and it depends on the SPECIAL stat of the opposing pokemon. The Slowpoke the the trainer uses has the hex code of 21 same as Slowpoke's special stat which is why Mew appears every time you battle the Youngster's pokemon. After you battle a trainer (after teleporting or flying) battle a Ditto but make sure the pokemon your using has a SPECIAL stat of 21 because if Ditto transforms it also assimilates the SPECIAL stat of the trainers pokemon. After you faint it you will be able to encounter a Mew that way. Also I caught a Dragonite by battling a Biker that has a LV 33 Weezing on the cycling path just in case your interested.

TheOmegaDragon : I already knew about Mew glitch, but that is really cool though isn't it? Lol a Dragonite, that must have been something. Was this (ur game) on emu, or game cartridge?

UserMike : I did it on both my cartridge and the one in the RGR, you can catch a bunch of pokemon that way. I caught Scyther, Magmar, Lickitung, Pinsir, Nidoking, Nidoqueen, Chansey, Arcanine, Lapras, and many more that way. I'm pretty much obsessed with the Mew Glitch on these games.

I never knew Mew was obtainable in Pokemon Yellow . ! I thought that Pokemon Yellow fixed all the glitches Pokemon Red and Blue had . Like the Missingno glitch . I loved that one in Pokemon Red/Blue . I duplicated the heck out of my master balls and rare candys lol .

That is interesting, I didn't know that, well, now I do!

BNuge : There is also an extra to this Glitch. It involves Ditto and the Mew Glitch. If it isn't known, I can post it here, with a complete list of the statistics.

I'll try this when I have a chance to but it's pretty risky since you may not be able to restore your game if you make a mistake.

lightless :

But if you're playing in the RGR, then you can avoid the risk by just not saving at all until the glitch is successful. Before you start, press F2. If something goes wrong just press F4 to retry it. That removes the risk.