Let the mashed potatoes cool down to room temperature. Store it in an airtight container and keep it in the fridge. Your mashed potatoes last for 3 to 5 days that way. You can also freeze mashed potatoes. Transfer it to a freezer-safe resealable bag. Take out as much excess air as you can and pop it in the freezer. This will extend the shelf life of your mash for up to a month.