The official MacDonald's Secret Sauce recipe is not available online. However, Todd Wilbur's recipe that is available on the Top Secret Recipes website replicates the sauce.
Mix half of a cup of mayonnaise, 2 tablespoons of French dressing, 4 teaspoons of sweet pickle relish, 1 tablespoon of minced white onion, 1 teaspoon of white vinegar, 1 teaspoon of sugar and one-eighth of a teaspoon of salt. Stir all the ingredients together in a bowl. Cover the container, and allow the mixture to refrigerated for several hours so that the flavors mix. This recipe makes three-quarters of a cup of sauce.