Spritz water on the mirror with a spray bottle to prevent mold spores from becoming airborne. Wipe the mold spores off with a paper towel. Discard the paper towel in a sealed trash can.
Fill a bucket with 1 gallon of warm water. Add 2 tablespoons of ammonia.
Dip a clean cloth in the diluted ammonia solution. Wring out the cloth to remove excess moisture.
Wipe the surface of the mirror with the ammonia-dampened cloth. Rinse the cloth as mold stains lift off the mirror. Add more ammonia solution and continue to clean the mirror to remove remaining mold spores and stains.
Dry the mirror with another cloth. Buff the glass to a shine with a second cloth, if desired.