Stir until sugar dissolves. Boil uncovered for 5 minutes.
Stir in lemon juice, lemon zest, and mint leaves. Remove from heat, cover, and let stand 30 minutes.
Strain lemonade into a 2 quart pitcher; stir in remaining 2 cups of water.
Cover with plastic wrap and let chill at least 2 hours.
To serve, fill pitcher with ice cubes, stir in extra lemon slices and mint sprigs, if desired.
Makes 1 1/2 quarts.