Spread some of the mint syrup on a small plate and turn the edge of a glass until it is evenly moistened. Add the sugar to a second plate and press the wet edge of the glass to the sugar, creating a green sugar rim.
Place the ice in the glass. Pour in the lemon juice and mint syrup. Stir well. Pour in the tonic water and stir again. Place the lemon slices on the rim of the glass and serve garnished with mint.