On June 17, 1967, the municipality of San Guillermo was created by virtue of Republic Act No.

Due to a noticeable increase in the rate of population migration and potential economic growth, additional barangays were created during the term of Mayor Alvaro Filart, such as barangays Burgos, Dietban, Estrella, Rizal, San Rafael and Villa Remedios.

Poverty Incidence of San Guillermo Source: Philippine Statistics Authority[13][14][15][16][17][18][19][20] As a municipality in the Province of Isabela, government officials in the provincial level are voted by the electorates of the town.

San Guillermo, belonging to the sixth legislative district of the province of Isabela, currently represented by Hon.

Faustino A. Dy V.[22] The Schools Division of Isabela governs the town's public education system.