Established in 1578, Agoo is considered the oldest town in La Union and was formerly part of Pangasinan. The town's name is believed to have come from the pine-like "aroo" trees that once lined its riverbanks.

Agoo played a significant role in early evangelization efforts in Northern Luzon, with Franciscan missionaries founding a church dedicated to St. Francis of Assisi. Over time, other religious orders such as the Augustinians contributed to the community's development by introducing education, religious practices, and construction techniques.

One notable religious structure is the church that was later elevated to the status of Basilica Minore of Our Lady of Charity in 1981. Agoo also witnessed conflict during the Japanese occupation in World War II, including tragic events in Barangay Macalva. In the postwar years, the town experienced continued growth and infrastructure development, supported by various civic leaders who helped promote its cultural and historical significance.

Like most of the LGUs in the Province located along the National Highway, Agoo, being a picture of sustainable progress and development, has become a town that never sleeps. Buses bound for Baguio City coming from Metro Manila, Western Pangasinan and Olongapo City and vice versa via Aspiras Highway have kept the Agoo Town Center busy 24 hours a day. Adding more economic activities are buses going to Ilocos Sur and Norte from Manila and back along the National Highway. Agoo is around 236 kilometers away from Manila, requiring about four and a half hours drive by land. Jeepney terminals located at the town proper offer transportation for those going to the interior areas of Tubao, Pugo and Santo Tomas towns. Its tricycles lined at the main town crossing also offer 24-hour transportation service.