Combine the potatoes, garlic and a pinch of salt in a pot. Add cold water to cover. Bring to a boil, reduce heat and simmer until the potatoes are tender when pierced with a fork, 15 to 20 minutes. Drain in a colander.
To the same pot, add the butter, milk, and some salt and pepper. Heat over medium heat until the milk is steaming and the butter melted. Add the potatoes and garlic back to the pot and mash until smooth. Fold in the sour cream. Taste for seasoning and add salt and pepper as needed.