Mix sugar and 1 cup water in a small saucepan over medium heat. Cook until sugar is dissolved, about 2 minutes. Remove syrup from heat.
Bring 4 cups water to a boil in a medium pot over high heat. Add tea bags and mint to pot; turn off heat. Steep for 15 minutes. Strain into a large bowl or pitcher, pressing on solids before discarding them.
Add syrup, lemon juice and 4 cups cold water to tea. Garnish each glass with mint leaves, if desired.