Place potatoes, carrots, and garlic in a medium saucepan. Cover with water. Add a large pinch of salt. Bring to a boil, and cook until completely tender, about 20 minutes.
Drain vegetables, reserving 3/4 cup cooking liquid. Return all but 1/4 cup liquid to pot. Add milk and butter, and season with salt and pepper. Mash until smooth, adding remaining liquid to desired consistency.