If you store fresh button mushrooms properly, they will stay good for about a week. Place whole, unwashed button mushrooms in a brown paper bag and then fold the top of the bag over. Place the bag in your refrigerator's main compartment. This works because the bag absorbs moisture from the button mushrooms, so they don't get moldy or soggy.
Avoid placing button mushrooms near foods with strong flavors or odors, as they will absorb them like a sponge. Also, be sure not to stack other food items on top of them, as bruised or squished button mushrooms won't keep very long. They also don't look very appetizing. If you don't have a lot of space in your fridge, you can dry the mushrooms. Dried button mushrooms can be re-hydrated, and they only take up little space in your pantry.