Preheat oven to 400 degrees F. Spray an 8 by 8 by 2-inch baking pan with nonstick vegetable cooking spray. Drain all but 2 tablespoons liquid from Mexicorn. Place reserved 2 tablespoons liquid in a medium bowl. Add egg to liquid and whisk to blend. Stir in Mexicorn. Add corn muffin mix and stir until just combined.
Transfer mixture to prepared pan.
Bake 20 minutes or until a toothpick inserted into center comes out clean.