Portabella mushrooms will generally last up to five days in the fridge if they are stored correctly. To ensure that your portabella mushrooms stay fresh and flavorful, it is essential to store them properly.  Set them in a paper bag or wrap them loosely in damp paper towels before placing them in the refrigerator. Be sure to check on the mushrooms regularly and use them as soon as possible to ensure the best flavor and texture.  Additionally, if you store cooked portabella mushrooms, be sure to fridge them within two hours of cooking.