Freshly mashed potatoes tend to be either light and fluffy or smooth and creamy, depending on how you prepare them. After sitting in the refrigerator overnight, however, mashed potatoes lose their original consistency and become clumpy, cold, stiff and dry. Reheating the mashed potatoes in the microwave will fix the temperature but leave them as clumpy and dry as before. As a result, you must take several extra steps when reheating mashed potatoes in the microwave. With some extra care, your reheated potatoes will be almost as good as they were before you refrigerated them.
Place the cold mashed potatoes into your microwave-safe container and stir them with your fork to loosen them and break up any clumps.
Sprinkle the mashed potatoes with a few drops of water, then cover the container with plastic wrap. This will help keep the moisture within the potatoes rather than allowing them to become even drier.
Microwave the mashed potatoes for 30 seconds on high, then remove from the microwave and stir with the fork. Replace the plastic wrap and return to the microwave for another 30 seconds. Repeat this process until the mashed potatoes are heated to your liking.
Add butter or milk to taste to the mashed potatoes and stir thoroughly to blend. This will add extra liquid and creaminess back into the mashed potatoes. As this will cool the mashed potatoes slightly, you may need to return them to the microwave to reheat them one last time after blending in these ingredients.