Preheat oven to 350 degrees F (175 degrees C). Grease bottom of 9x13-inch baking dish.
Stir brownie mix, oil, water, and eggs together in a bowl until well-blended. Spread mixture in prepared baking dish.
Bake in preheated oven until a toothpick inserted 2 inches from the side of the dish comes out almost clean, about 25 minutes.
Transfer baking dish from oven to wire cooking rack and while still hot, arrange mint candies across the top about 1/4- to 1/2-inch apart. Let sit until candies melt, about 3 minutes. Spread melted mints evenly over brownies with a spatula. Cool until top layer hardens, 4 to 5 hours.