Prepare meatloaf: Whisk together milk, bread crumbs, eggs, minced onion, salt, and sage in a large bowl.
Stir in ground beef and mushrooms until well combined.
Shape mixture into a round loaf. Place loaf into a 5-quart slow cooker.
Cover and cook on Low until a meat thermometer reads 160 degrees F (71 degrees C), 5 to 6 hours.
Prepare glaze: Whisk ketchup, brown sugar, mustard, and Worcestershire sauce in a small bowl.
Open the slow cooker and spoon glaze over the meatloaf. Cover the cooker and continue to cook on Low until sauce is heated through, about 15 minutes.
Carefully remove meatloaf with two spatulas to a serving plate. Cover with foil and let stand for 10 minutes before cutting.
