Born at Owando, Ondongo belongs to the Kouyou ethnic group. He began working at the Marien Ngouabi University in Brazzaville in the 1980s. After Denis Sassou Nguesso returned to power in the June–October 1997 civil war, he appointed Ondongo as Adviser to the President on the Economy, Finance, and the Budget; Ondongo served in that post from 1997 to 2002. He was then appointed to the government on 18 August 2002 as Secretary of State for Budgetary Reform and Financial Administration, working under the Minister of the Economy, Finance, and the Budget. He was promoted to the position of Minister of Labour, Employment, and Social Security on 7 January 2005.