Move the couch from your home to an outside area. Mold spores emit potentially toxic fumes, and the spores themselves can spread to other locations over time. For best results, place the couch in direct sunlight until it dries. This may take several days.
Use a hand-held vacuum or hose extension to remove mold spores, and then clean the vacuum filter outdoors. Vacuum all creases and crevices in the areas where mold appears.
Mix a solution of equal parts white vinegar and water, and dip a sponge in the solution. Begin scrubbing the leather vigorously wherever you see mold. Vinegar makes a handy leather cleaner and a strong mold killer. Wait about 15 minutes before you move on to the next step.
Dab the leather couch with a damp sponge to remove the vinegar. Dry the leather by leaving the couch in the sun or by applying direct heat, like the heat from a blow dryer or a small space heater.
Observe your leather couch over the next few days. If mold begins to re-emerge, this can signify that there is mold far beneath the surface of the leather. As a result, the couch, or at least one or more of the cushions, will require replacement.