It is possible to remove mildew from leather by brushing off as much of it as possible with a clean, dry brush and then following up with a little soapy water and leather conditioner. Doing the cleaning outside prevents the spread of mildew spores within the home.
A clean, soft sponge dipped in a little bit of the soapy water is enough to wipe away whatever mildew remains after brushing. It is important to allow the leather to dry completely in a spot that is away from heat, sunlight and dust. Once dry, a leather conditioner used according to the maker's directions often restores the proper look and feel of the leather and protects it from further damage.