If the holder is asleep, it will consume its held Mint Berry and be cured of sleep. In Pokémon Stadium 2, if the holder moves last and is put to sleep by a move like Sleep Powder, it will consume the Berry so it can attack; on the other hand, if the holder moves first and uses Rest, the Berry will activate at the end of the turn (this protects the holder from attacks like Spore or Thunder Wave, but makes it vulnerable to Dream Eater before the Berry is used).
Bag item
When used from the Bag on a Pokémon, it cures that Pokémon of sleep. This consumes the Mint Berry.