Cut the top off the head of garlic, exposing the cloves.
Drizzle 1/4 cup olive oil over the exposed cloves.
Wrap in aluminum foil and roast in preheated oven for 40 minutes.
Bring a large pot of salted water to a boil. Add potatoes and cook until tender but still firm, about 15 minutes; drain.
In a small saucepan heat milk and butter until butter is melted. Using a potato masher or electric mixer, mash potatoes until smooth. Stir in warm milk and butter mixture. Mash in roasted garlic cloves.