[3] Originally, Chillán Viejo was the location of a Spanish fort with the name of San Ildefonso, which was established in 1565, during the campaign of Pedro de Villagra against Loble and the Mapuche north of the Biobío River.

It acquired some fame during the Chilean War of Independence, especially in the Siege of Chillan from July 15 to August 12, 1813, after the death in her (May 21 of that year) of the Spanish leader, Antonio Pareja, and it was sacked by the royalist montoneras of Pincheira on September 18, 1819.

[4] According to the 2002 census of the National Statistics Institute, Chillán Viejo spans an area of 291.8 km2 (113 sq mi) and has 22,084 inhabitants (10,791 men and 11,293 women).

As a commune, Chillán Viejo 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 by Victor Pérez Varela (UDI) and Mariano Ruiz-Esquide Jara (PDC) as part of the 13th senatorial constituency (Biobío-Coast).