Heywood Fleisig received bachelor's degrees in economics, philosophy, and political science from Swathmore College (Swathmore, Pennsylvania) in 1961. In 1963, Fleisig obtained a Master of Arts. in economics from Yale University. He later finished his PhD in economics at Yale University, specializing in international finance, trade, and economic history in 1969.

Prior to joining the World Bank, Fleisig served as a Coffee and Cocoa Analyst for Merrill Lynch, Pierce, Fenner, and Smith in New York City from 1961 to 1962. He also taught economics as an Assistant Professor at Cornell University from 1966 to 1974. From 1974 to 1979, Fleisig served as an Economist for the Division of International Finance of the Federal Reserve Board. He later served as the Principal Analyst for the Fiscal Analysis Division of the Congressional Budget Office from 1979 to 1982.

In February 1982, Heywood Fleisig began his World Bank career as an Economist for the International Trade and Capital Flows Division of the Economics Program Department (EPDIT). Fleisig served in numerous roles at the World Bank, including: Senior Economist in the Global Analysis and Projections Division of the Economics Program Department (EDPGL), January 1984 to May 1986; Senior Economist in the East Asia and Pacific Vice Presidency (AENVP), December 1986 to May 1987; Principal Economist in the Asia Vice Presidency (ASIVP), July 1987 to May 1989; Principal Economist in the Country Operations Division, Asia-Country Department II (AS2CO), July 1989 to January 1990; Principal Economist in the Office of the Chief Economist, Latin America and Caribbean Region (LACCE), January 1990 to May 1993; and Adviser in the Private Sector Development Department (PSD), October 1993 to March 1996. Fleisig retired from the Bank in 1996, but later served as a consultant to the International Finance Corporation (IFC) and World Bank Investment Climate Department (CIC) in 2005 and 2007.

After his retirement from the Bank in 1996, Fleisig served as Research Director for the Centerfor the Economic Analysis of Law (CEAL) located in Washington, D.C.