For the syrup, in a small saucepan combine sugar and 1 cup of water over high heat; bring to a boil. Cook until sugar dissolves. Remove from heat; let cool completely.
In a blender, combine mango with about 3 cups of water and blend until smooth. Pour through a strainer into a pitcher. Add 3 more cups of water. Stir in lemon juice and syrup, mixing well. Add about 4 cups of ice to pitcher.
Pour into chilled glasses, garnish with mint if preferred, and serve.