Best Way to Reheat Mashed Potato
No matter which method you choose, add fat and liquid. Cold potatoes have lost moisture. They need a hug. A splash of warm milk, a knob of butter, or even a drizzle of olive oil will resurrect them.
Also: never reheat mashed potatoes with garlic or cheese already in them. Those flavors can turn bitter. Instead, add fresh garlic powder or shredded cheese after reheating. Trust me on this one.