In a small saucepan, bring sugar, water, and fresh mint to a simmer until sugar is dissolved, then simmer for 2-3 additional minutes until the syrup begins to get a slight tint to it. Remove from heat, and taste test. Let sit to further infuse for a more intense flavor.
Strain the liquid into an airtight container to remove any particles and the leaves. Store in an airtight container - like a glass jar - in the fridge. It should last for 2-3 weeks if stored properly.