The early settlers and occupants called it "Lapo", short for "Lapo-Lapo", a tall grass which grew everywhere in the area.

In the year 1772, Don Tomas Aquino, the first local leader, initiated the organization of the scattered communities into one, calling the place "Lapog", a derivative of local vernacular "Lap-Lapog" which means unirrigated land.

For 189 years, "Lapog" remained in the records as the official name of the town until June 18, 1961, Republic Act No.

It is bordered by Cabugao to the north, Magsingal to the south, the provinces of Ilocos Norte and Abra to the east.

Poverty incidence of San Juan Source: Philippine Statistics Authority[11][12][13][14][15][16][17][18] San Juan, belonging to the first congressional district of the province of Ilocos Sur, is governed by a mayor designated as its local chief executive and by a municipal council as its legislative body in accordance with the Local Government Code.