While cleaning a moldy, dirty leather seat may not be a pleasant job, it’s a necessary one if you want to keep the seat. Left untreated, the mold may spread and finally discolor or damage the leather. All the loose mold and mildew spores have to be taken away outdoors before you are able to thoroughly clean out the chair.
Enlist a friend that will help you carry the seat outdoors, if necessary. Wear a dust mask and rubber gloves to prevent contact with the mold. Removing the mold outdoors, as opposed to inside, prevents loose spores from affecting indoor air and surfaces.
Remove all loose cushions from the seat. Note the wind direction, and stick near the chair so the wind blows away from the seat and away from you. Brush the seat from top to bottom with a soft-bristled brush like a natural-fiber paintbrush. Brush all the crevices. Brush all sides of each cushion too, propping the cushions up against a wall or vertical surface to maximize airflow.
Flip the seat on its side or upside down, whichever makes the bottom area more accessible. Set cardboard or a plastic tarp on the floor under the seat to protect the upholstery if the ground is not soft. Inspect this place for mold and brush down it also if you become aware of any suspicious specks or spots. Flip the seat back erect.
Plug a vacuum cleaner with HEPA filter to an extension cord rated for outdoor use. Vacuum the entire seat from top to bottom, including the room reserved for cushions. If the cushions are permanently attached, use a crevice tool to vacuum the gaps between cushion and chair. If the cushions are removable, vacuum all sides of each cushion too.
Flip the seat over and vacuum the bottom to remove dust and loose debris. Flip it back erect.
Mix equal parts water and rubbing alcohol in a spray bottle. Spray the moldy leather using the alcohol solution, or pour some of the liquid onto a lint-free white rag. Wipe the mold away using the white rag, replacing it as it appears dirty. Spray and wipe any mold you find on the bottom of the seat or at the region under the cushions, or about the cushions, also.
Mix 1 part white vinegar, 2 parts olive oil in a spray bottle. Replace the lid and shake the bottle to mix the liquids.
Wipe down the leather with a slightly damp white rag, and then spray a minimal quantity of the vinegar and oil solution over the damp leather, using as small as possible in each place. Rub the whipped leather using a fresh damp cloth, working the solution to the upholstery. Continue spraying and buffing until you’ve cleaned all the chair’s leather. Wipe it dry using a lint-free white towel.
Allow the seat to air out at a covered outdoor area such as a porch or garage for several days to eliminate lingering smells. If you can’t leave it outdoors, set the seat at a well-ventilated, non-humid room, then using a fan for air circulation.