Beat the Elite 4 12 times. Then, go to Cerleuan Cave. Go to the Island where Mewtwo was. There will be a wild Mew there.
OR...
Beat the Elite 4 12 times. There will be a Mew in the place where Zapdos was in the Power Plant.
OR...
Trade from Emerald, and how you get it in Emerald is it will be at Faraway Island by having the Old Sea Map.
OR...
I got onto a file for a code to get a Mew in LeafGreen. How? If you enter a code from the "file", you will be able to get it by using the walk-through walls code from the place where you saw Loreli in Icefall Cave. There walk forward on the water, past the blocking rocks (which you will get through by walking through walls code), and Mew will be on a mini island
OR...
Go to the S.S Anne Truck (find it out on YouTube), use strength on the truck and Mew will be hiding on the side of the truck
OR...
THINK!
It will be too bad if you cannot get Mew.
P.S I will not tell you the "file" code.
P.P.S You need a Missingno to get Mew while using the trick in the Power Plant
trade it in from emerald
You can't get mew in firered or leafgreen.
mew and mew two
mew and mew two
There is no Mew 3.
Mewtwo? Plus you cannot get Mew in Leafgreen or Firered.
You can't catch mew on leafgreen you need gameshark codes for that
Mew two
No.
Mewtwo, yes, if you have a FireRed or LeafGreen with Mewtwo.
Mew has to be traded from emerald and ho-oh you have to get from Pokemon colosseum.
Cheatcodes or luck.
Mew was distributed at an event once, but that is long over and sadly if you want a mew in leafgreen now, you'll need help from other games or you could use cheat codes.