Place the mango in the freezer for 30 minutes to allow it to cool. If you are in a hurry you can skip this step and add 3-4 ice cubes into the smoothie instead. Remove the skin of the mango with a food peeler, Cut mango into medium chunks, reserve about 1 teaspoon of mango to use later for garnish.
Place the mango, frozen banana, milk, Greek yogurt and 1/4 teaspoon cinnamon in the blender. Blend for about a minute or until the mixture is creamy. Use a spoon to scrape the edges of the blender if any mango gets stuck on the sides. Add more milk if needed. Pour into a cup, top with the reserved mango and garnish with a sprinkle of cinnamon or mint leaves.