Castillejos was founded in 1743, making it one of the earliest established towns in the province of Zambales and older than nearby Subic, which became a pueblo in 1769. Historical accounts indicate that the first settlers were Tagalog families, most likely from the province of Bataan. While some initially settled along the coast, frequent pirate attacks along the shoreline forced many families to move farther inland toward the mountains. This shift in settlement allowed the community to grow steadily and securely, eventually developing into a recognized pueblo under Spanish rule.

The town was originally known as Uguic (Uguit). For more than a century, it functioned as a “visita,” meaning it was spiritually dependent on Subic. A priest residing in Subic would travel on scheduled days to Uguic to administer sacraments such as baptisms and marriages not only for its residents but also for nearby communities. This arrangement continued until January 23, 1863, when Uguic was finally granted its own parish priest, marking the end of its religious dependence. On that same significant day, the town adopted the name Castillejos, symbolizing a new era of ecclesiastical and civic identity.

During the Spanish colonial period, the town was administered by local officials known as gobernadorcillos, later referred to as capitanes municipales. These leaders rendered public service without salary, often making personal sacrifices for the welfare of the community. Archival records from the early 1800s show that agriculture formed the foundation of the local economy. Residents cultivated coconut, cacao, pepper, palay, mongo, and cassava, reflecting a hardworking agrarian society.

In the 19th century, the territorial jurisdiction of Castillejos was considerably larger than it is today. However, as neighboring communities such as San Marcelino and San Antonio gained independence and became separate pueblos, the land area of Castillejos was gradually reduced. Boundary disputes with Subic were formally addressed in 1897, when representatives of both towns met to define and mark their territorial limits, agreements that were later approved by provincial authorities.

During the American occupation, Castillejos was consolidated with neighboring towns for administrative efficiency under Act No. 945. However, governance challenges and strong local advocacy for self-rule led to the eventual restoration of its political independence in 1910. From that time onward, the municipality continued to progress under successive presidents and municipal mayors.

The name Castillejos was inspired by the historic Battle of Castillejos (1860) during the Spanish-Moroccan War. The victory of Spanish forces under General Juan Primo y Prats earned him the noble title “Marqués de los Castillejos.” Two years after his elevation to nobility, the town of Uguic adopted the name Castillejos in honor of that celebrated event.

Today, Castillejos stands as a municipality rich in history and heritage. From its early beginnings as Uguic, through periods of religious dependence, territorial changes, colonial transitions, and eventual political independence, the town’s story reflects resilience, unity, and the enduring spirit of its people.