If you have any sautéed mushrooms left over, store them in an air-tight container in the refrigerator. For the best flavor, use them within 3 days, but they should keep for up to 5 days. To reheat mushrooms, preheat the cooking pan over medium heat first. Then, add the mushrooms with the juices and butter that might be in the storage container. Cook it just until mushrooms are heated through.