Place the ground beef, breadcrumbs, egg, onion, milk, salt, and pepper in a large mixing bowl. Mix with your hands until the ingredients are evenly combined but not overworked.
Shape the mixture into an oval loaf roughly 2 inches thick and set it in the bottom of the slow cooker.
Stir the ketchup, brown sugar, and mustard together in a small bowl until the sugar dissolves and the glaze is smooth.
Spread the glaze evenly over the top and sides of the meatloaf.
Cover the slow cooker and cook on low for 6 hours, or until the internal temperature reaches 160°F. Let the meatloaf rest for 10 minutes before slicing.