Move your furniture outdoors if possible. This will prevent the spread of mold spores. Always wear gloves and a protective mask when dealing with mold.
Wipe off as much of the mold as possible with a soft bristled brush. You can also vacuum it off with a shop vacuum.
Combine an equal amount of rubbing alcohol with water in a container. Soak a lint free washcloth in the solution, and wring out the excess.
Test the solution on an inconspicuous area to ensure that it will not harm the leather.
Wipe the mold off the leather furniture with the washcloth. Move the furniture back indoors out of the sun to prevent it from fading. Allow the furniture to air dry.