Place potatoes, garlic, rosemary, and thyme into a large pot and cover with salted water; bring to a boil. Cook, covered, until potatoes are easily pierced with a knife, about 30 minutes. Drain, reserving 1 cup cooking water.

Transfer boiled potatoes and garlic to a large bowl; discard rosemary and thyme. Add olive oil, salt, and pepper. Mash with a potato masher, adding cooking water to reach desired consistency.