Remove as much visible mildew from the couch as possible by buffing any affected areas with a clean, dry cloth or small, nylon-bristled brush, knocking loose mildew to the floor. Vacuum up the loose mildew pieces and throw away or empty the vacuum bag.
Combine 1 cup of either isopropyl alcohol or distilled white vinegar and 1 cup of water. Mix well, until the two are thoroughly combined.
Pour the solution into a plastic spray bottle.
Test the solution in an inconspicuous spot to be sure it will not cause streaking or staining on the leather prior to applying the mixture to the sofa. Spray the test area lightly and then wipe the solution away with a soft cloth. Wait for the treated area to dry completely and then evaluate the area. If no discoloration or damage is apparent, continue.
Spray the sofa with the dilute alcohol or vinegar solution. Be sure to cover all portions of the couch, paying particular attention to the corners as mildew thrives in dark places.
Wait five minutes and then wipe any residual solution away with a dry cloth. Any alcohol or vinegar aroma will dissipate over time. Wait 48 hours to assess the effectiveness of the treatment and repeat if necessary.
Treat persistent odors by placing a shallow dish filled with coffee grounds in the center of the couch cushions and then covering the sofa with sheets of plastic. Tuck the edges of the plastic beneath the sofa, sealing the coffee inside. Leave the couch covered for three to five days and then remove the plastic. Dump the coffee into the trash and run a vacuum over the leather to remove any loose grounds.