Naguilian geographically positioned at the center of La Union, with a total land area of 10,086.85 hectares, making it the third-largest among the 19 towns and cities in the province.

The name "Naguilian" is derived from the Iloco word nagilian, which is based on the root íli, meaning "town."

The prefix nag- indicates the present or past tense, while the suffix -an expresses a perfective aspect, signifying a completed action or state.

[5] In its early history, Naguilian was originally a settlement of Cordilleran tribes, particularly the Kankanaey and Ibaloi people, who established themselves along the Balili River and the foothills of the Cordillera Central.

The settlements traded goods such as gold, silk, textiles, rice, porcelain, wax, and earthenware with neighboring lowland Ilocano (Samtoy) and Pangasinan communities, as well as foreign merchants from China, Japan, and Maritime Southeast Asia.

[7] During the Spanish colonization in the 1700s, Ilocano immigrants began settling in La Union's coastal areas, including the establishment of the town of Bauang.

Due to population growth, settlers followed the river inland and established a colony in a fertile valley they named "Naguilian."

[5] They cultivated the land, focusing on agriculture, particularly rice, corn, and sugarcane the primary ingredient in producing traditional wine called "basi" and vinegar.

[8] On October 29, 1849, Governor-General Narciso Zaldua Clavería issued a decree unifying eight towns from Pangasinan, three from Ilocos Sur, and territories in the Cordillera to create a new province called La Union.

On March 2, 1850, Governor-General Antonio María Blanco signed the Superior Decreto, officially establishing La Union.

Under the leadership of General Manuel Tinio y Bondoc, allied with Emilio Aguinaldo, locals fought against Spanish forces in Northern Luzon.

Revolutionary leaders from Bauang, such as Remigio Patacsil and Mauro Ortiz, played significant roles in the liberation efforts, which led to the eventual withdrawal of Spanish forces from La Union.

On the quiet morning of January 23, 1945, the small sitio of Mangkaeng, nestled in the forested hills of Guising Norte, became the site of an unimaginable tragedy.

Japanese soldiers stationed in the town proper of Naguilian were part of the rear guard for General Tomoyuki Yamashita, retreating toward the Cordillera mountains as Allied forces pressed on.

[9][10] Determined to eliminate any potential resistance or witnesses, the soldiers moved into Mangkaeng, where families from nearby villages such as Imelda, Natividad, and Ortiz had gathered, believing they had found safety in the remote sitio.

Some attempted to flee into nearby creeks, hoping the dense forest would offer cover, but the soldiers were relentless, pursuing and killing those who tried to escape.

The massacre left Mangkaeng silent, its forest floor stained with the lives lost in one of the darkest days in the town's history.

The Municipality of Naguilian has a total land area of 10,086.85 hectares, making it the third largest among the nineteen towns and cities in the province of La Union.

The Naguilian River, located in the western portion of the town, originates from the province of Benguet and flows westward through La Union, eventually emptying into the Lingayen Gulf and South China Sea.

The upper reaches of the river are classified as Class A by the Environmental Management Bureau, indicating suitability as a water source for drinking, while the lower reaches have lower water quality and are categorized under Class C. The second major waterway springs from the Municipality of Burgos and flows through the valleys of Barangays Aguioas, Sili, Al-alinao Norte, Nagsidorisan, Angin, Suguidan Norte, and Natividad.

The municipality is susceptible to hydro-meteorological hazards, including flooding, rain-induced landslides, and tension cracks, particularly during intense typhoons brought on by heavy monsoon rains.

The majority of Naguilian's population is Ilocano, with indigenous groups such as the Kankanaey and Ibaloi primarily residing in the upland areas.

[19] Naguilian is predominantly Roman Catholic, though various other Christian denominations, including Iglesia ni Cristo, Protestant groups, and Jehovah's Witnesses, are also present.

These fertile areas benefit from the natural irrigation provided by the water sources, making them ideal for various forms of agriculture.

In recent years, efforts have been made to improve farming practices, enhance irrigation systems, and diversify crops in order to boost productivity and sustainability in the agricultural sector.

The town is renowned as the home of the original Basi wine-making industry, a traditional Ilocano wine made from sugarcane juice that dates back to the World War II era.

Other industries include metal craft, ceramics or brick-making, concrete hollow block production, and furniture making and upholstery.

Poverty incidence of Naguilian Source: Philippine Statistics Authority[23][24][25][26][27][28][29][30] Naguilian, belonging to the second congressional district of the province of La Union, 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.