In a medium saucepan, combine the potatoes and whole garlic; add enough cold water to cover by about 2 inches. Bring to a boil over high heat. Reduce heat to a simmer, and cook until the potatoes are easily pierced with a fork, about 15 minutes. Drain the potatoes, and set aside in a warm place.
Meanwhile, in the same saucepan, combine milk, butter, and salt; season with pepper. Place over low heat until butter has melted and milk is warm to the touch. Pass potatoes and garlic through a ricer or food mill into saucepan, and stir gently to combine with milk mixture. Serve immediately.
To make ahead (up to two hours), complete recipe, then top with a thin layer of milk. Cover; set aside in a warm place. When ready to serve, stir to combine.