Add all ingredients to a medium saucepan and heat over medium heat, stirring occasionally.
Bring to a boil, then remove from heat and allow the mixture to steep for 90 minutes.
After 90 minutes, strain the mint leaves from the syrup, squeezing any excess syrup out of the mint leaves as you strain it. Discard the pulp and transfer the mint syrup to a syrup bottle or other container. Store in the fridge for up to 2 weeks.
Notes
This recipe makes just over 1.25 cups of mint simple syrup!