1984-1986: David started out as a research assistant for the University of Los Baños Program on Environmental Sciences and Management. In 1985, he became a part-time research assistant for the UPLB-NEDA Philippine Institute for Development Studies on Migration.
1986–1987: He was an Instructor to UPLB’s Department of Social Sciences and Graduate School.
1989: He became Development Management Officer for the Department of Environment and Natural Resources (DENR) Upland Development Program.
1992: He became Chief of the Strategic Planning-Planning and Policy Studies Office of DENR. In the same year, he became Chief of the Research and Monitoring Staff of the Liberal Party-PDP LABAN for the 1992 Presidential Elections.
1993-1995: He became executive assistant and Committee Secretary or the Supervising Legislative Staff Officer under the Office of Senator Francisco Tatad. In 1995 to 1996, he became director of the Political Affairs Staff of Senator Tatad.
1996-1998: He served as Senior Executive Assistant of the Kaunlaran Group of Companies.
1997-1998: He then became Consultant on Political Affairs and Head of the Political Operations for Mindanao under the Office of Deputy Speaker Hernando Perez at the House of Representatives.
1998-2004: David then transferred to the office of Sen. Robert Jaworski as Head Executive Assistant and Concurrent Director for Political Affairs.
2007-2008: He was a part-time faculty on Environmental Science in Enderun Colleges in Pasig City.
(from ABS CBN)