Master Ball/Core series games/Effect

Manual activation
When used from the Bag in a encounter, it catches the wild Pokémon without fail (with the exception of the ghost Marowak in Pokémon Tower).

In Pokémon: Let's Go, Pikachu! and Let's Go, Eevee!, throwing a Master Ball will bypass the Poké Ball throwing mechanic, and will instead show a cutscene of the Master Ball catching the chosen Pokémon. If used on Legendary Pokémon in these games, they will attempt to smack the ball away, but will still be caught anyway.

The Master Ball cannot be used in situations in which Poké Balls cannot be used, such as in wild battles with two or more opponents currently present or against a trial Pokémon. If used in a Trainer battle, the opposing Trainer will bat the ball away, wasting the item.

As a Pokémon's Poké Ball
From Generation VI onward, bred Pokémon inherit their Poké Ball from their parents. However, the Master Ball and Cherish Ball cannot be passed down via breeding; instead, they act as a regular Poké Ball for inheritance purposes.

Held item
will fail if used while holding a Master Ball.