Agoncillo is one of thirty-one (31) municipalities (along with three cities) that comprise the province of Batangas in the Calabarzon Region. The town is politically subdivided into (21) barangays, and is a part of the 3rd Congressional District of Batangas.

The town of 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 through the provincial board, requesting the Secretary of the Interior, Malacañang, to detach and separate 11 barrios from Lemery to be created into a separate municipality.

By virtue of an Executive Order issued by then 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 Elpidio Quirino, lifting the suspension and thereby, authorizing the immediate organization of the Municipality under the name of Agoncillo, in honor of Don Felipe Agoncillo, a native of Taal and one of the first Filipino representatives to the Spanish Cortes.