Put sugar, water, sliced apples, and cinnamon in a medium saucepan and bring mixture to a boil.
Once boiling, lower heat and let simmer for about 15-20 minutes, until mixture has thickened and apples have softened.
Strain mixture into a bowl, being sure to press down on the apples 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. Syrup will continue to thicken as it cools.