Monster Mayhem: Build and Battle is a trainer game where the player creates and customises a monster using various components and colors. Once built, the player navigates their monster through a city from a bird's-eye view, seeking ingredients to create tonics and consuming Gruffins - small rabbit-like creatures that boost the character's statistics when eaten.
The main exploration area has several battle arenas around its edge, where enemy monsters will spawn intermittently. Entering these battle arenas while a monster is present changes the perspective to a one-on-one fighting game shown from a side view. Battles are resolved via a rock, paper, scissors combat system with the addition of tonics to perform special attacks.
The Wii version of the game supports motion controls, increasing the power of the attacks. This feature adds an extra layer of interaction and immersion to the gameplay experience.