Place sugar and water in a medium saucepan; stir together, and bring to a boil over high. Boil, stirring often, until sugar is dissolved, about 1 minute. Remove from heat, and stir in mint leaves. Set aside at room temperature until completely cooled, about 1 hour. Pour into an airtight container; cover and chill 24 hours. Remove and discard mint. Store, covered, in refrigerator up to 1 week.