Add sugar, water, and basil leaves to a 2-quart (or larger) saucepan and bring to a simmer over medium heat. Stir continuously until sugar is dissolved, about 5 minutes. Remove from heat and let syrup cool in pan for 1 hour.
Strain basil leaves from syrup. Set aside while you make the lemonade, or refrigerate syrup in a covered container until ready to use.
Combine lemon juice, 5 cups of water, and the simple syrup in a large pitcher. Stir to combine.
To serve, add ice cubes, lemon slices, and 1/2 a basil sprig to a glass and fill with lemonade.