Put water, sugar, and diced mango in a medium saucepan and bring to a boil.
Once boiling, lower heat and let mixture simmer for about 15 minutes until mangos are softening and mixture has thickened (it will continue to thicken as it cools).
Strain syrup into a bowl, pressing down on the mango with a wooden spoon or spatula to extract as much juice as possible.
Keep syrup in bowl or pour into a mason jar and place in fridge to cool completely before using.