San Isidro was originally part of the Municipality of Allen (formerly Minapa-a and La Granja).
The first human beings that have landed on Calagundian shores around the last quarter of the 19th century were some intrepid Bicolanos, Boholanos, Leyteños and Cebuanos pioneers.
Later, during this period, other groups of immigrants were lured from the nearby island provinces and Bicol regions to the coastal plains of Calagundian.
These families formed the nucleus of Barrio Calagundian, a part of La Granja town (now Allen).
Finally, after years of political struggle for independence from its mother town Allen, the municipality of San Isidro was established by virtue of Republic Act No.
These were San Roque, Sabelia, Happy Valley, Acedillo, Alegria, Salvacion and Mabuhay.
[7] Yearning a separate parish from that of Allen, Northern Samar, a church decree dated October 31, 1959, made it realized.
Its patron saint is San Isidro de Labrador whose feast is religiously celebrated every 15th day of May.
The topography of the land is characterized from plains to rolling hills and steep mountain ranges.
The lowlands are along the coast but in a few sections of this area, the mountains rise from 30 to 61 meters (98 to 200 ft) high above sea level.
Majestic Mawo River traverses the eastern portion of the hinterlands finding its way at the town of Victoria before discharging into the Samar Sea.
As to its bedrock foundations, old sediments and pyroclastics which are more stable than poorly consolidated sediments (e.g. limestone, shale or alluvial deposits) in terms of compressive strength/load bearing capacity comprise roughly 67.50% of the total municipal land area.
Mineral production consists mainly of non-metallic such as sand, gravel, earth and rocks.
The low level ground classified as “A” comprised a meager 3.06% of the total land area and is located at the eastern and western sections of the municipality.
Approximately, its land uses were distributed as follows: San Isidro is politically subdivided into 14 barangays.
The coastal barangays, starting from the south are Caglanipao, Veriato, Mabuhay, Palanit, San Juan, Salvacion, Alegria, Balite, Buenavista, Poblacion Sur, Poblacion Norte, and San Roque while the interior barangays are Seven Hills and Happy Valley.
The total number of households in the municipality during the 2000 Census of the Philippine Statistics Authority accounted to 4,184.
Based on the Municipal Health record, total number of birth as of 1995 registered to 526 or a rate of 2.29%.
With reference to the records of the Philippine Statistics Authority, majority of the household population of the municipality spoke Waray language (50.59%) mostly during their early childhood.
The Iglesia ni Cristo registered third with 582 followers or about 3.17% Seventh Day Adventist ranked fourth with 230 or 11.25%.
Of the 5,989 economically active populace (in the labor force), 5,400 (49.49%) were employed registering 4,811 higher than the unemployed (589 persons or 5.4%).
The proportion who were literate was slightly higher among males, registering a figure of 6,833 or 51.62% than females which accounted to 6,394 or 48.38%.
The proportion, on the other hand, that was academic degree holders accounted to 328 or 2.25% of the total household population 7 years old and over.
However, due to the residents' clamor - particularly the parents and the students for a four-year course, bachelor's degree on BS in Agriculture Education was subsequently offered at the same school through the extension program of the University of Eastern Philippines in June 2001.