Fill your sink or bowl with a bit of iced water. Transfer the container or Ziploc bag into the water, ensuring that it is only partially submerged.
Take a ladle or spoon and gently stir the mashed potato to facilitate faster heat loss. You want to get the mash down to a temperature of under 70°F.
Another way of doing this is to use a cooling paddle. If going down this route, it’s best to transfer the mashed potato into a wide bowl.
Once the temperature is below 70°F, you can transfer the potato into a shallow container or Ziploc bag and place it in your fridge, still open, until the temperature drops to 40°F.
Once it does, you can then put the lid in place or zip the Ziploc bag closed and leave the mash in the fridge until you want to use it.