In a small saucepan over moderate heat, bring the sugar, 1/4 cup water, lemon juice and zest to a boil and cook until the sugar has dissolved and the mixture has reduced by half. Chill in the fridge at least 1 hour and up to 24 hours. Right before serving, stir in the mint and pour over the mangos and berries. Garnish with whole mint leaves.
BYOC: You will love this syrup so much you will be pouring it over fruit all year long. For something a little different, try using fresh basil instead of the mint. This also makes a fantastic base for a drink. Try a mojito by adding a bit of rum and club soda. Or try adding fresh ginger for a kick of warm spice.