He is known for his work on behavioral economics, which is a field that combines economics and psychology to understand how people make decisions. Mullainathan has made significant contributions to the understanding of how cognitive and emotional factors influence economic behavior and has developed novel methods for analyzing these factors using big data and machine learning.

Mullainathan is also known for his work on development economics and has conducted extensive research on poverty and inequality. He has worked with a number of international organizations, including the World Bank and the International Monetary Fund, to develop and implement policies aimed at reducing poverty and promoting economic development.