Mashed potatoes take some effort so it’s rarely worth making them for just one or two servings. That’s why freezing them can be handy. Once your leftover mashed potatoes have cooled, put them in a freezer-safe container or freezer bag, and remove as much air as you can. They will be fine for up to a month. You can then thaw them in a pan on the stove or in the oven at 350°F for about 35 minutes.