Combine lemon juice, 1 cup water, and brown sugar in a saucepan over medium heat. Stir until sugar dissolves, 2 or 3 minutes. Remove from heat and let cool. When cooled to room temperature, pour into a pitcher.
Pour tonic water, cold brew concentrate, and 1 1/2 cups water into pitcher; stir. Refrigerate until chilled, about 1 hour.
