Heat oven to 350°F. Line a 9 x 13-inch pan with foil and coat with cooking spray. Squeeze out the liquid from a thawed box of chopped frozen spinach. Place spinach, vegetable oil, and carrot juice in blender and puree.
In a large bowl, combine family-style brownie mix, eggs and the spinach mixture. Stir until well blended. Spread batter in prepared pan and bake 40 minutes.