Add the diced potatoes, onion, garlic, chicken broth, salt, and pepper to the slow cooker. Stir to combine.
Cover and cook on low for 7 hours or until the potatoes are very soft.
Use a potato masher to break up some of the potatoes for a thicker texture, leaving some chunks.
Stir in the milk, then add half of the shredded cheddar cheese and half of the crumbled bacon. Stir until the cheese melts.
Turn off the heat and stir in the sour cream until fully combined.
Ladle the soup into bowls and top each serving with the remaining cheddar cheese and bacon.