Combine sugar, 1 cup water in a small saucepan; bring to a boil. Remove from heat and stir in mint leaves. Let stand for 30 minutes and then strain out mint leaves.
In a 2-quart pitcher, combine mint-infused simple syrup, remaining water, lemon and mandarin juices; chill overnight.
Serve over ice and garnish with a sprig of mint, if desired. Makes 2 quarts.