Divide the mint evenly into an ice cube tray and pour the key lime juice evenly over top. Freeze until key lime juice is completely frozen.
Crush the key lime-mint ice cubes in a blender. Divide mixture among 4 large margarita glasses. Top each glass with 8 ounces club soda and stir gently to combine. Garnish each with a sprig of mint and serve immediately.
Recipe courtesy of Juan-Carlos Cruz