To unlock Mirror Mode in Mario Kart World, a series of specific in-game achievements must be completed:

• Complete all Grand Prix Cups at 150cc (not necessarily in first place).
• Complete all Knockout Tour Cups at 150cc (just need to place in the top four, not necessarily first).
• Collect at least 10 P Switch missions in Free Roam.
• Find at least 10 Question Mark (?) Panels in Free Roam.
• Collect at least 10 Peach Medallions in Free Roam.
• After completing these, replay the Special Cup at 150cc. After the credits, a post-credits cutscene will confirm the unlocking of Mirror Mode.

Once Mirror Mode is unlocked, it will be available in all major racing modes including Grand Prix, Knockout Tour, and Versus Mode. Additionally, Mirror Mode can also be activated in Free Roam by going to Peach Stadium, driving onto Peach's castle roof, and driving through the stained glass window to flip the world.