Fresh mushrooms can be frozen, according to About.com. Before freezing, mushrooms should be washed and cooked. Mushrooms can be sautéed or steamed before storing in the freezer.
Before cooking the mushrooms, wash them in cold water. After washing, trim the mushroom stems. Any mushrooms larger than an inch can be quartered or sliced before cooking, while smaller mushrooms can be left whole. Sauté the mushrooms in a skillet for several minutes or steam them for up to five minutes. Once the mushrooms are cooked, let them cool and place them in an airtight container. Frozen mushrooms can stay good for up to a year.