Insert a wire brush into the crack on the prefab fireplace brick. Move it gently in a back and forth motion to remove any debris.
Mix a batch of mortar in a container, adding water according to the directions. Fill in the crack on the fireplace by scooping up a small bit of mortar with a trowel and pressing it into place. Smooth the mortar by running over it with the trowel.
Moisten the mortar by misting with water twice each day for one week during the curing process.
Paint the mortar with a shade of cement paint that matches the brick in your prefab fireplace.