Cook macaroni per package instructions and cool to room temperature.
In a mixing bowl, add mayonnaise, milk, brown sugar, vinegar, mustard, salt, and pepper.
Whisk those ingredients until smooth.
In a large mixing bowl, combine macaroni, carrots, onion, celery, and green onions.
Pour the dressing over the macaroni and stir until everything is cooked evenly.
Refrigerate for at least 1 hour to let all the flavors marinate.