Put the sugar in a small saucepan with the lemon juice, 1 mint leaf, and 2 tablespoons water. Bring to a boil and remove from the heat. Stir until the sugar is well dissolved. Remove the mint leaf and add the cinnamon. Let cool.

Cut the mangoes into halves, running a sharp knife around the pits to detach. Peel and slice thinly. Arrange the slices on 4 serving dishes and drizzle with the sugar syrup. Cover and refrigerate for 15 minutes. Decorate with mint leaves when ready to serve.