Its name is the plural form of the Spanish mejillón meaning "mussel", referring to a particularly abundant species and preferred staple food of its indigenous inhabitants.

To the west, in the northern part of peninsula, is Punta Angamos [es], the site of the naval combat of the same name, fought during the War of the Pacific (1879-1883).

Also after the War of the Pacific, administrative organization began, which in the case of Mejillones would involve the preparation of urban planning in agreement with the existing type of port.

The plan, which was perhaps more realistic than those proposed before, was based on 63 city blocks and was completed with the Foundation Act (Acta de Fundación), signed by president Germán Riesco and dated February 7, 1906.

In 1970, a majority vote approved the bill presented by Mayor Araya Cuadra to build a theater located across the rear of the Plaza Almirante Latorre.

Such funding would not satisfy the completion of the project, and work stopped in 1972 pending the granting of a loan extension that failed to materialize by the time of the coup of the Armed Forces of September 11th of 1973.

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