Add sugar, light corn syrup and lemon juice to a small saucepan (like like to add some of the lemon pulp as well). Heat and stir until sugar is dissolved and mixture just starts t o boil. Removed from heat immediately.
While syrup (mixture from above) cools a bit, puree watermelon and partially tear the mint leaves.
Whisk together lemon syrup and puree, then add mint leaves. Pour mixture into one-gallon Ziplock bag or large glass measuring bowl. Seal bag or cover bowl and let it sit (steep) in refrigerator until chilled.
Note: You can adjust the strength of the mint by how long you let it steep. I do about 4 hours and the mint is noticeable, but not overpowering.
When ready to churn, strain mint leaves from the sorbet mixture and pour into ice cream maker. Churn until sorbet is a thick slush and hardly spins.
Pour sorbet into 1 quart container and let it solidify in the freezer for at least 4-6 hours.
Let sorbet thaw a few minutes before scooping and serving.