El Cajon, Spanish for "the big box," was first recorded on September 10, 1821, as an alternative name for sitio rancho Santa Mónica to describe the "boxed in" nature of the valley in which it sat. The name appeared on maps in 1873 and 1875, shortened to "Cajon," until the modern town developed in which the post office was named "Elcajon."
In 1905, the name was once again expanded to "El Cajon" under the insistence of California banker and historian, Zoeth Skinner Eldredge.
In the 1960s and 70s, Frontier Town, Big Oak Ranch, was a tourist attraction, featuring a typical frontier town themepark and a periodic simulated shootout. The park closed around 1980 and is being used for residential housing.