Ordinarily, Mr Mime is a regional exclusive in Europe, where it will spawn in the wild.
It has, however, since its debut in 2016 appeared in various events so other players have had access in their own region.
One such opportunity is during the Tour Kanto event in February 2021, which tasks players with catching all 150 Pokémon within the span of eight days.
During this event Mr Mime - alongside fellow Kanto exclusives Farfetch d, Tauros and Kangaskhan - appears in Raids on the main event day on Saturday, February 20th.
At the time of writing, this is the only way to get Mr Mime before the Raid Collection Challenge deadline on February 28th, 2021 - so be sure to prioritise catching it on the day before the opportunity disappears.