It is the oldest commercial building in Southern California,[2][3][4][5][6] and was one of the first ten sites in Los Angeles County to be listed on the National Register of Historic Places, receiving the recognition in 1971.

[7][11] The pine and sycamore beams are tied with leather thongs,[12] and the structure is also supported by large buttresses which can still be seen on building's exterior corners.

[13] Water was brought to the mill in an open ditch (zanja) from Los Robles and Kewen Canyons,[11] and stored in a large cistern.

[13][16] The mill was operational for only seven years, during which time it provided food for the missionaries and Indian neophytes, there were 1,644 Tongva-Gabrieleños in 1816[7] (Population of Native California) in the mission community.

[16] In 1846, Pío Pico – last Mexican governor of Alta California – sold 16,000 acres (65 km2), including the mill, to Julian Workman and Hugo Reid (co-grantee of the adjacent Rancho Huerta de Cuati).

[7][8] With title to the land in a state of uncertainty, James S. Waite (publisher of The Star newspaper) established squatter's rights over 160 acres (0.65 km2), including the Old Mill.

[7] In 1898, Los Angeles Times reporter Topsy Tinkle wrote a lengthy article following a visit to El Molino Viejo.

[10] Tinkle described the condition of the mill as follows: The grinding-stones have gone, and also the machinery that in the romantic time of the old mission padres and their Indian neophytes, was wont to turn their corn into meal, and yet, in the material of the building itself, no sign of decay.

The large oak beams, only ten inches apart, as sound as in the day the original trees lifted their leafy tops high in air.

When the Huntington Hotel opened in 1914 on the nearby hill, the land around the mill was turned into a golf course, with El Molino Viejo as the clubhouse.

The millstones were found more than a century after the mill closed on the grounds of the Huntington Library by General George S. Patton, who grew up in the area and recalled seeing them used as blocks for mounting horses.

[13] In his 1898 publication, Topsy Tinkle recounted a story that the Indians told about a natural spring located on the site of the Old Mill.

The story told of a 16-year-old Indian named Catalina with "thick, jet-black hair" and "big, melting black eyes.

"[10] As the oldest commercial building in Southern California, El Molino Viejo has been recognized as a historic site at the state and national levels.