Significant colonial settlement of the region did not take place until the 18th century when priests from the Church of the Holy Spirit of Esparza settled in the area.

The priests had been traveling with indigenous peoples from the Salamanca region towards Cartago and stopped in what was then known as the Valley of Three Rivers (Spanish: Valle de los Tres Ríos).

Their presence in the area attracted permanent settlement and a stable population, and the fledgling town became known as Nuestra Señora del Pilar de los Tres Ríos (English: Our Lady of the Pillar of the Three Rivers).

[7][8] Due to dynamic urban growth in the metropolitan area of Costa Rica in the late twentieth century, the land use has progressively changed from agricultural to suburban development.

However, regions within La Unión, including Tres Ríos, Concepción and San Ramón, continue to grow coffee beans for the gourmet market.