For the mint syrup: Combine the sugar and 1/2 cup water in a small saucepan and bring to a boil. Cook until the sugar has melted. Stir in the mint and let steep 30 minutes. Strain the mint leaves.
For the cocktail: Combine the bourbon, tea, lemon juice, lime juice and 2 ounces mint syrup in a cocktail shaker and shake for 5 seconds.
Rub the orange slice around the rim of highball glass and fill with ice. Pour the cocktail over the ice and garnish with the mint sprig.