Place the sugar, water, and mint leaves in a small saucepan and bring the mixture to a boil. Let it boil for 2–3 minutes, until the sugar dissolves. Strain the simple syrup into a jar and let cool.
Put 4 cups ice, the bourbon, mint simple syrup, and lemon juice into a blender and pulse until smooth and slushy. Pour into goblets or highball glasses and garnish with mint leaves.