Dennis is a co-founder of GlobalGiving and was its CEO until the end of 2010. GlobalGiving is the world's leading marketplace that connects donors directly to community-led economic and social development projects around the world. During Dennis's tenure, GlobalGiving mobilized funding to nearly 4,000 projects in 109 countries around the world from 140,000 individual donors and many of the most innovative Fortune 100 companies.

From 2007 to 2010, Dennis was Lead Economist and Senior Partner of the World Bank's Strategy and New Products Units, where his teams created the Innovation and Development Marketplaces, which have now been replicated in over seventy countries. His teams also created and ran the first Strategic Forums and Corporate Days for the Bank's president and senior management team. In addition, Dennis was the primary author of the Bank's Strategic Directions Paper and oversaw the allocation process for the Bank's $1.2 billion budget in 2009 and 2010.

Earlier in his World Bank career (1986-2007), Dennis worked primarily in Indonesia and Russia. He worked on and led teams that prepared several multi-hundred million dollar loans and numerous economic and policy analysis studies. In 1984 and 1985, Dennis worked at the Asian Development Bank and USAID in Manila Philippines.

Dennis has a BA in religious studies with honors from UNC-Chapel Hill (1983), where he was a Morehead Scholar. He did his graduate work in development studies and economics at the Woodrow Wilson School, Princeton University, where he received his MPA in 1986. In 2000, he completed an Executive Development Program at Harvard Business School.