Absolutely, mashed potatoes are great for freezing leftovers. Keep the potatoes in an airtight, freezer-safe container, and they should keep for two months. To thaw, place the frozen mashed potatoes in the refrigerator overnight or thaw on the defrost setting on your microwave. As the mashed potatoes thaw, they may seem watery, especially using the microwave, but keep stirring as you heat them, and they should come back together nicely.