Agoncillo was originally a part of
Lemery. In 1945, an executive committee was formed by the first appointed Mayor Jacinto Mendoza Sr. to prepare a resolution, requesting the Secretary of the Interior,
Malacañang Palace, through the provincial board to detach and separate 11 barrios and be created a municipality. Through
Executive Order 140 issued by
President Elpidio Quirino, the
Municipality of Pansipit was created. But the Municipal Council of Lemery passed a resolution requesting for the revocation and suspension of the said creation. Thus, another executive order was endorsed for the conduct of a plebiscite to ascertain the true sentiments of the residents regarding the issue of separation. Finally, on April 17, 1949,
Executive Order No. 212 was issued by President Quirino, lifting the suspension and thereby, authorizing the immediate organization of the Municipality under the name of Agoncillo. ==Geography==