Add watermelon to a blender and pulse until well pureed (there should be about 4 cups). Pour through a fine mesh strainer into a bowl.
In a large pitcher whisk together water, lemon juice and sugar until sugar has dissolved. Stir in pureed watermelon. Stir in ice and mint (alternately add ice and mint directly to individual cups and pour lemonade over). Store in refrigerator.
*If you don't have time to chill the ingredients then just use more ice in place of some of the water.