You can encounter and capture Mantine in the waters of Sevii Island’s Seven Island. Complete the Islands quest to enable new regional Pokémon to appear on the map and use Surf around the bodies of water of Seven Island to encounter Mantine.

Getting to Seven Island

Mantine can only be found in Seven Island, and is one of the Johto-native Pokémon to appear in Gen III games after you finish Celio’s gemstones quest. Take note however that Mantine is exclusive only to Pokémon LeafGreen and cannot be found in Pokémon FireRed.

Start your journey to Seven Island by heading to the port in Vermillion City.

Present your Rainbow Pass to ride the Seagallop Ferry to the island.

Once you get to the island, head to the shore of the water area where your Pokémon can use Surf as some of the water surrounding the island has fast-moving currents which prevent you and your Pokémon to surf across.

Mantine can be found and encountered as you ride on Pokémon surf back and cannot be caught via fishing. Its level will be around Level 35-40, so you can have your lead party Pokémon at around this level, and then use Repel to ward off weaker Pokémon.

Mantine has a 5% encounter rate, so it might take a while before you find him. With enough patience, you should be able to encounter it after a few tries.

Catch Mantine as you would any other Pokémon. Make sure to weaken it first, or apply status effects to make it easier for you to catch him. It is weak against Electricity, Grass and Bug-type Pokémon, so you can use that to your advantage.

Once weakened, fling your Poké balls at him until you have caught Mantine.

Congratulations on catching your first Mantine! Its move set when caught will be as follows:

Mantine does not have an evolved form, but nevertheless is a pretty cool Pokémon to add to your Pokédex collection especially if you are on a completionist run!