In a small saucepan, bring sugar, syrup, lemon juice, grenadine and mint leaves to a boil dissolving the sugar granules. Let cool slightly. Cut the rind off the watermelon and cut into chunks.
Puree half of the chopped watermelon in a blender. Strain through a fine mesh strainer into a large glass bowl. Repeat with remaining watermelon. Add slightly cooled syrup mixture and mix well to incorporate. Place sorbet into a 2-quart plastic container and freeze for 3 to 4 hours.