Cut watermelon into 1” pieces and put into a blender. Six cups (¼ of watermelon) should yield around 2 cups of juice. Blend the watermelon and strain into a measuring cup.
Add strained watermelon juice to a saucepan and simmer until reduced to 1 cup.
Add sugar and stir to dissolve.
Remove from heat and add mint. Steep for 10 minutes.
Strain into storage container.
Notes
Simmering the watermelon juice to 1 cup will take around 45 minutes. Stir occasionally. Adding the mint at the end is key. If added sooner, it will add a bitter vegetal taste to the syrup.