La Unión was founded in 1821 during the government of Bernardo O'Higgins, to secure sovereignty over the Central Valley south of Valdivia.

The commune is covered in forest to the west of the city and the east is dominated by agricultural landscapes spanning Los Llanos.

[3][4] According to the 2002 census of the National Statistics Institute, La Unión spans an area of 2,136.7 km2 (825 sq mi) and has 39,447 inhabitants (20,125 men and 19,322 women).

[2] As a commune, La Unión is a third-level administrative division of Chile administered by a municipal council, headed by an alcalde who is directly elected every four years.

The commune is represented in the Senate during the 2014–2022 period by Alfonso de Urresti (PS) and Ena von Baer (UDI) as part of the 16th senatorial constituency (Los Ríos Region).