Place potatoes, bouillon, and garlic in a large pot and cover with water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 15 minutes.
Drain in a colander over a large bowl, reserving cooking water.
Mash potatoes with sour cream and cream cheese, adding reserved water as needed to reach desired consistency.
Transfer mashed potato mixture to a slow cooker, cover, and cook on Low for 2 to 3 hours.
Just before serving, stir in butter and season with salt and pepper to taste.