[3] According to legend, it was first named Angela in reference to her husband, Mariano Kalingog, whose family was one of the foremost settlers of the place.

Angela drowned while she was washing clothes along the Pinacanauan River, bringing grief to her husband, who also died some years later.

In 1920, by virtue of Executive Order 25, San Mariano became a Municipal district, and with the passage of Philippines Legislature Act No.

[6] In 1985, three councilors from Barangay Ibujan in the municipality of San Mariano, including Ibanag community leader Luis Gabriel who had rejected overtures for the establishment of a local CHDF base in the town, were forcibly taken by heavily armed men who claimed that they needed the three as guides.

[7] When the German naturalist Carl Semper hiked the Sierra Madre mountains in May 1860, he observed early inhabitants who are known today as the Kalingas.

He described the said group as a typical Filipino "cultural minority" who grew their own food, practiced their own religious rites, smithed their own tools, decorated their own artifacts with distinctive designs and traded forest products for metal and salt.

They inhabited the forest close to the "Kalinga" settlements and showed up with games in seasonal periods to barter for agricultural products.

The once verdant forest slowly became barren and denuded with the ceaseless illegal logging and kaingin system employed by the local people and new migrants.

It is bounded on the north by the Ilagan, on the east by Palanan, on the south by San Guillermo and on the west by Benito Soliven.

About 29,264 hectares or 19.91 percent are presently devoted to extensive agricultural activities with corn, rice and bananas as the major crops.

Poverty incidence of San Mariano Source: Philippine Statistics Authority[14][15][16][17][18][19][20][21] As a municipality in the Province of Isabela, government officials in the provincial level are voted by the electorates of the town.