Born in Lemery Batangas, Agoncillio obtained a bachelor's degree in philosophy from the University of the Philippines in 1934 and a master's degree in the arts from the same university the following year. He earned his living as a linguistic assistant at the Institute of National Language and as an instructor at the Far Eastern University and the Manuel L. Quezon University. In 1956, he published his seminal work, Revolt of the Masses: The Story of Bonifacio and the Katipunan, a history of the 1896 Katipunan-led revolt against Spanish rule and its leader, Andres Bonifacio. He garnered acclaim for this book, as well as criticisms from more conservative historians discomforted by the work's nationalist, perhaps even Marxist bent.
In 1958, Agoncillo was invited to join the faculty of the Department of History of his alma mater, the University of the Philippines. He remained with the university until his retirement in 1977, chairing the Department of History from 1963 to 1969. Philippine President Diosdado Macapagal named Agoncillo as a member of the National Historical Institute in 1963. He served in this capacity until his death in 1985.
Agoncillo's History of the Filipino People, first published in 1960, remains to date a popular standard textbook in many Filipino universities, as are many of Agoncillo's other works. While Agoncillo was and still is controversial for his perceived leftist bent, his works stand well to the right in the ideological spectrum as compared to his equally influential contemporary Renato Constantino. At the same time, Agoncillo's perspective is considerably more liberal than that of Gregorio F. Zaide. Zaide, Agoncillo and Constantino stand as the most prominent 20th century Filipino historians to emerge during the post-war period. It must be noted however, that Agoncillo's works suffer from uneven scholarship throughout, especially with his use (or especially, non-use) of reliable historical sources.
Filipinos in History: Volume III, National Historical Institute (Manila, 1996), pp. 6-7.