When you can smell mildew, you have mildew, whether you can see it or not. If your mattress smells of it, the mildew is either on the mattress or nearby. Mildew odor is musty and damp, and doesn't provide a healthy, relaxing place in which to sleep. Removing the smell requires that you remove any traces of mildew on the mattress and in the room, as well as create a climate in the room that does not promote the return of mildew and its accompanying odor.
Vacuum the surface of your mattress on the top, bottom and all sides to remove surface mildew spores. Throw out the bag to avoid mildew transferring from inside your vacuum to somewhere else. Run a dehumidifier in your room, if needed, to rid the room of excess humidity.
Take your mattress outside and set it in direct sunlight. Allow the mattress to remain outside until completely dry, rotating it to another side every couple hours. If weather doesn't permit this, turn on an electric heater in your room to help dry out the mattress.
Mix 2 tablespoons mild dishwashing detergent in 1 gallon of warm water. Stir the two together, vigorously with a spoon, to create frothy soap suds. Dip your clean sponge in the suds only, not the liquid, and blot at the mildew stains on your mattress to kill any remaining mildew.
Stir together 1 cup each rubbing alcohol and water. Wet a cloth with the solution and sponge any remaining mildew stains. Removing the mildew stains is important in removing the accompanying odor.
Allow your mattress to dry completely. If you can do this in the sunlight, it speeds up the drying process and helps freshen your mattress faster. If not, allow your mattress to dry inside with the aid of open windows or an electric heater.
Sprinkle baking soda over your mattress. Rub the baking soda into the surface with a clean, dry cloth. Allow the baking soda to absorb odors and deodorize your mattress overnight before vacuuming it up in the morning.
Spray disinfectant spray over the surface of your mattress to kill any lingering mildew spores and odor. Let your mattress air out for a few hours before making the bed.