The formal history of the municipality dates back on February 11, 1953 when President Elpidio Quirino proclaimed the area as a settlement project by virtue of Proclamation No. 368 creating the Economic Development Corp (EDCOR) under the control and supervision of the Armed Forces of the Philippines (AFP). It mandates to reserve about 8,920 hectares of untapped forest land in the province of Isabela specifically within the territorial jurisdiction of the municipalities of Angadanan and Echague. Before the proclamation, the place was inhabited by “AETAS” who wandered through the forest, making their way along creeks and rivers. Its vast and rich forest resources have lured logging concessionaires to invest in timber logging business ventures. As a result, roads were constructed in many parts of the reserved area for easy access of forest products to and from the market in the town of Angadanan and Alicia where industrial sawmills are located. This major development paved the way for other people to migrate and cultivate potential agricultural land in the area.
Considering the potential economic growth and through the unified effort of the populace, the said area was created into a new barangay named Barangay Peredo Edcor in honor of Col. Wilfredo Peredo, Commanding Officer of the AFP in the area.
In 1954, the National Resettlement and Rehabilitation Administration (NARRA) was created by virtue of R.A. No. 1160 which took over the administration of the reservation and the Economic Development Corp. The creation of NARRA further bolster the development in the area by expanding the scope of recipients of the program to include settlements of surrendered dissidents, ex-servicemen in the military and landless civilians recruited from the different regions of the country. On June 30, 1967, by virtue of the Memorandum signed by the National Defense and Land Authority, the Administration of the reservation including that of Barangay Peredo Edcor was transferred to the Land Authority. And since then, the whole area, NARRA & EDCOR FARM was known as “ISABELA SETTLEMENT PROJECTS”.
In June 17, 1967, the Municipality of San Guillermo, Province of Isabela was created by virtue of R.A. No. 4906 sponsored by Congressman Melanio T. Singson, Representative of Isabela. San Guillermo was named in honor of the late Eugenio S. Guillermo, who was then the mayor of Angadanan, Isabela. Among the barangays taken from the municipality of Angadanan were: 1. Anonang, 2. Colorado, 3. Calaoagan, 4. Dipacamo, 5. Peredo Edcor, 6. Guam, 7. Nakar, 8. Palawan, 9. Progreso, 10. San Francisco Norte, 11. San Mariano Norte, 12. San Mariano Sur, 13. Villa Rose, 14. Villa Sanchez, 15. Villa Teresita. Barangays taken from municipality of Echague were: 1. Aringay, 2. Dingading, 3. San Francisco Sur and 4. Sinalugan.
Later on, because of noticeable increase in number of population migration and the potential economic growth of the municipality, additional barangays were created during the incumbency of Mayor Alvaro T. Filart. The following barangays were created namely: 1. Burgos, 2. Dietban, 3. Estrella, 4. Rizal 5. San Rafael and 6. Villa Remedios. Subsequently barangay Peredo Edcor was divided into two (2) barangays namely: Centro 1 and Centro 2, which is now the poblacion and the official seat of the Municipal Government with twenty six (26) barangays comprising the whole municipality of San Guillermo.
The government both national and local concerted their efforts in bringing economic development in the area by providing essential infrastructure projects and the delivery of basic services to its constituents.
Today, the Municipality of San Guillermo is categorized as 4th income class municipality with 26 progressing barangays. Its vision and trust is for a more improved quality of life among its constituents and making them more effective and dynamic partners in political governance, and socio-economic development in the country as a whole.