Add the ketchup, brown sugar, vinegar, and mustard to a small bowl.
1 cup ketchup, 1 ½ Tbsp brown sugar, 1 tsp dry mustard, 1 tsp apple cider vinegar
Stir with a fork or small whisk until the brown sugar has completely dissolved and the glaze is smooth. It should be glossy and pourable, about the thickness of warm honey.
Pour the desired amount of glaze over the top of your formed raw meatloaf and spread it to the edges with the back of a spoon.
Bake the meatloaf according to your recipe.
Let the meatloaf rest 10 minutes before slicing so the glaze can set.
You can make additional sauce for serving on the side.