Dennis has worked for 30 years in international aid and philanthropy. He co-founded GlobalGiving, the first global crowdfunding website, and was CEO from 2000 to 2010. GlobalGiving has mobilized $225 million for nearly 15,000 projects in 165 countries, fueled by hundreds of thousands of individual donors and 130 leading companies and foundations. From 1986-2000, Dennis was an economist at the World Bank, where he worked in Indonesia, Russia, Papua New Guinea, and Niger on agriculture, housing reform, energy efficiency, structural adjustment, and innovation.

His New Products Team created the Innovation and Development Marketplaces in the late 1990s. He has also worked at USAID and the Asian Development Bank, been Executive Chairman of Ashoka Changemakers, served as Visiting Lecturer at Princeton University, Professor of the Practice and Entrepreneur in Residence at UNC-Chapel Hill, and Visiting Fellow at the Center for Global Development. He is a graduate of UNC-Chapel Hill, where he was a Morehead Scholar, and the Woodrow Wilson School at Princeton.