Before joining the Gates Foundation, Stonesifer spent two decades working in technology. She was a consultant to DreamWorks SKG and held several vice president positions at Microsoft. She began her tenure running Microsoft Canada before being promoted to vice president and revamping Microsoft's Product Support operations. Later she oversaw the Consumer Products Group and served as senior vice president of the Interactive Media Division. In the latter position, Stonesifer was responsible for an $800 million business whose product was interactive entertainment, news, information and service products. In addition, Stonesifer managed Microsoft's investments in new online content and service products, including MSN. The Interactive Media Division produced software titles including Microsoft Encarta Encyclopedia, Microsoft's "Magic School Bus" Series and Microsoft Flight Simulator. In 1996 she negotiated a Microsoft and DreamWorks SKG joint venture, DreamWorks Interactive, which was subsequently acquired by Electronic Arts.