Appointed by President Clinton to serve on the bipartisan Commission on Entitlement and Tax Reform in 1994, Greenstein frequently testifies before Congress on domestic public policy. In 1981, he formed the Center on Budget and Policy Priorities, a nonpartisan organization committed to public policy and budget analysis and improving the lives of poor families.

Greenstein, who was named a MacArthur Fellow by the John D. and Catherine T. MacArthur Foundation in 1996, received a bachelor's degree from Harvard University and attended the London School of Economics and the University of California, Berkeley.