Put water, sugar, and mint leaves in a medium saucepan, stir, and bring to a boil.
Lower to a simmer and let mixture simmer for about 10 minutes, until syrup has thickened.
Remove from heat and pour mixture through a fine mesh sieve to strain mint leaves out and into a heat-proof bowl or mason jar. Place bowl or jar in fridge to cool completely before using. Syrup will continue to thicken as it cools.