To activate a mirrored map in Free Roam, you can warp over to Peach Stadium and follow the track up and jump on to the roof of Peach’s Castle. Enter the iconic stained glass window of Peach and it’ll warp, sending you into a mirrored version of the map. You cannot do this in the pseudo-Free Roam multiplayer lobby.

Spotted by IGN, an account on X called Ninjinaut showcased the Mirror Mode Easter egg in Free Roam for Mario Kart World. The video sees Baby Rosalina jumping into the window, which is a direct callback to Super Mario 64's mechanic for accessing new worlds.