In a medium saucepan over medium heat, combine the sugar and water. Stir until the sugar dissolves. Do not let the mixture boil.
Remove from heat and add the mint leaves.
Let steep for 15–30 minutes, stirring occasionally. Steep longer for stronger mint flavor. (Peppermint: steep only 10-15 minutes.)
Strain into a clean glass jar through a fine-mesh strainer. For extra clarity, strain again through cheesecloth to remove fine particles. Refrigerate for up to 2 weeks or freeze for longer storage.