Prep The Simple Syrup. In a small saucepan, heat the sugar with 1 cup of water over medium-high heat until the sugar has totally dissolved. Set the syrup aside to cool slightly while you gather your other ingredients.
Make The Watermelon Juice. Add watermelon cubes to a blender and puree until it's as smooth as possible. Strain the watermelon juice through a fine mesh sieve to strain out any seeds or pulp.
Combine. Pour the strained watermelon juice into a large pitcher and add lemon juice, simple syrup, and 4 cups cold water. Stir to combine well. At this point, you can chill the watermelon lemonade in the fridge until you're ready to serve it.
Finish. Right before serving, add 3-4 cups ice cubes to the pitcher and garnish with lemon slices, watermelon balls or watermelon slices, and fresh mint leaves, as desired. Serve with plenty of ice!
Store leftover lemonade in the fridge 3-4 days.