Previously known as Puerto San Carlos, it was renamed La Unión on July 13, 1824, after El Salvador gained independence from Spain and formed the United Provinces of Central America.

Completed in November 2012, the port has since helped increase local economic growth and stability, though competition from nearby San Miguel remains strong.

At the end of the 18th century, what is now La Unión was categorized as a town and it was given the name in honor the King, who at the time was Charles III.

The most important products that have been grown are: coffee, cocoa, sugar cane, oleaginous seeds, fruits, and grass.

A temple named El Templo de Conchagua was built in 1693 and it rapidly became one of La Unión's tourist attractions.