Preheat the oven to 350°F and lightly grease your 9×13 baking dish.
In a large pot of lightly salted water, bring to a boil. Add macaroni and cook until just al dente, 8 - 10 minutes. Drain well.
In a large saucepan over medium heat, melt the butter. Whisk in the flour and cook for 1 full minute, whisking constantly.
Slowly whisk in the milk, garlic salt, dry mustard, and pepper. Continue cooking, stirring often, until the mixture bubbles and thickens.
Remove the mixture from the heat and stir the shredded Cheddar and the American cheese into the hot milk mixture until completely melted and smooth. The sauce should be a little thick.
Mix the cooked macaroni and cheese sauce directly in your 9×13 dish. Spread evenly. Cover with foil.
Bake for 20 minutes, uncover foil and bake for an additional 10-15 minutes, or until bubbly around the edges and lightly golden on top.
Let cool for at least 5 minutes before serving so the sauce can set up.