To make the mint syrup: Add 1 cup sugar and 1 cup water along with a handful of fresh mint leaves to a small saucepan. Cook over medium heat until the sugar dissolves and the mixture just starts to simmer. Cool, then strain into a jar. Store in refrigerator.
Add lemonade, club soda, and mint syrup to a glass over ice. Garnish with mint and a slice of lemon.
Notes
To make a pitcher: Use 4 cups lemonade, 4 cups club soda, and 1/2 cup mint syrup. Serves 8.