Mold can develop on any surface exposed to prolonged periods of moisture and warmth and this includes your windowsills. Cold air meeting your windows (which are warmer since your home is heated) condenses and runs down the window to pool on the sills. In addition, the sills tend to collect organic materials like dirt, dust, skin cells, plant pollens, etc. While you can clean and remove mold from a windowsill it’s best to prevent it from growing in the first place.
Here are some things you can do to prevent mold growth.
1) Keep your curtains or blinds open to allow circulating air to dry the moisture.
2) Use a dehumidifier in your home to remove excess moisture.
3) Apply a sealant to trim and windowsills to block moisture.
4) Get rid of the organic materials (dust, dirt, etc.) on the windowsills to deprive mold of its food source. Dust the sills a couple of times per week.
5) Lightly spray sills with MoldOff to prevent mold growth, do not wipe off.
If you are unlucky enough to still develop mold, don’t panic. MoldOff is a great mold destroyer as well as a preventer. Spray the sill and let the MoldOff stand for 24 hours. Wipe it down and, if necessary, repeat. MoldOff won’t strip paint or wood finish and destroys mold at the roots, not just on the surface.