Mewtwo locations — where to catch

In Lumiose City (Legends: Z-A)

• Lysandre Labs — Gift

In Johto & Kanto (HeartGold & SoulSilver)

• Cerulean Cave — Static Encounter

In Kanto (FireRed & LeafGreen)

• Cerulean Cave — Static Encounter

In Kanto (Red, Blue & Yellow)

• Cerulean Cave — Static Encounter

Other games (4)

• Sword / Shield — The Isle of Armor (Gen 8): Max Lair
• Let's Go, Pikachu! / Let's Go, Eevee! (Gen 7): Cerulean Cave
• Ultra Sun / Ultra Moon (Gen 7): Ultra Space Wilds, Ultra Space
• X / Y (Gen 6): Unknown Dungeon

Mewtwo — full Pokédex entry