The two best options for reheating mashed potatoes are in the oven or on the stove top. Either way, you'll want to add liquid like milk or half and half and optionally more butter. (You can also use broth or stock for a lighter option.) For every cup of leftover mashed potatoes, you should add 1 to 2 tablespoons of liquid. Reheat on the stovetop in a saucepan over medium heat, adding liquid gradually and stirring often until hot. To reheat in the oven, gently stir in liquid as directed above (and dot with butter if desired), then spread in an even layer in a baking dish. Cover with foil and reheat in a preheated 350˚F oven until hot, 20 to 35 minutes (depending on the amount) or until the internal temperature reads 165˚F on an instant read thermometer. Avoid the microwave for reheating mashed potatoes as it can dry them out.