Place the cubed potatoes, diced ham, and chopped onion into the slow cooker and stir to combine.
In a mixing bowl, whisk together the cream of chicken soup, milk, garlic powder, salt, and black pepper until smooth.
Pour the soup mixture over the potato and ham mixture and stir gently to coat everything evenly.
Sprinkle 1 cup of the shredded cheddar cheese over the top, then cover and cook on low for 6 hours.
Stir the casserole once more, sprinkle the remaining cup of cheddar cheese on top, and let it melt for 10 minutes before serving.