Mold build is a very common problem as purses and handbags age. With care, you can remove mold and mildew and get a few more years of life out of your favorite purse.
Let In Some Light
Remove all items from the purse and leave it open. Getting some fresh air and sunlight in the bag will help eliminate odor and kill some of the mold. The longer you can leave the purse out in the fresh air the better, but try to leave it out for a full day.
After the mold growth is neutralized, the purse should be wiped down with a some lemon juice diluted with water. The citric acid will kill any remaining mold. You may want to do this two or three times just to make sure you've gotten it all. The lemon juice is generally safe for leather and dyes, so there's no need to worry about ruining your purse.
Heavier Cleaning Products
If the mold build up is significant, you may need to use a specific mold cleaner. You want to read labels when buying these as you don't want to damage the leather or inadvertently remove dye. Avoid anything that has bleach in it as it will almost certainly damage the bag. LTT Em Clean from LT solutions is specially made to remove mold from leather goods. A more ecologically conscious option is Rustic Touch. It is safe and uses organic materials. Follow manufacturer's directions on how to apply the products.
Once your purse is mold free, practice habits that will prevent future build up. Mold can't survive in the sunlight, so periodically cleaning out your bag and leaving it in the sun is a good place to start. Also, keeping your purse free of perishable food items and liquids will help stop mold before it starts. Cleaning the bag with leather cleaner a few times a year is also good. You don't need to wait for mold to appear to clean your purse.