In a small saucepan, combine sugar and 1/4 cup water. Bring mixture to a boil, stirring until the sugar has dissolved. Remove from heat. Let stand until completely cool.

In a large pitcher half-filled with ice, add 2 cups water, lemon juice, simple syrup, mint, and lemon slices. Stir to combine. Serve over ice. Garnish with mint leaves and lemon slices.