Though there are varying accounts of the exact date, San Gabriel Mission records indicate it was built in 1816.
The mill was built on land owned by the San Gabriel Mission, and was designed by Franciscan Father José Maria de Zalvidea, then in charge of the mission.
It was built by Tongvan Mission Indian laborers  "under the watchful eye"  of Father Zalvidea.

The mill was built like a fortress.
Its lower walls are nearly five feet thick at the base, and are made of brick and volcanic tuff.
Some have written that the thick fortress-like walls were intended to allow the padres to barricade themselves in the event of "a disturbance among their somewhat uncertain converts."
The upper walls are made of sun-dried adobe slabs, and the building's surface is covered with a lime mortar made from burnt sea shells.
The pine and sycamore beams are tied with leather thongs, and the structure is also supported by large buttresses which can still be seen on building's exterior corners.

Water was brought to the mill in an open ditch (zanja) from Los Robles and Kewen Canyons, and stored in a large cistern.
For the mill itself, Father Zalvidea designed an unusual horizontal, direct impulse water wheel.
There were three vaulted water chambers on the ground floor and a single horizontal water wheel.
The water wheel was attached to a vertical shaft that went up to the second level (the current entrance room) where the grinding stones were located.
One of the grinding stones was attached to the shaft, and rotated along with the water wheel.
A second stone was placed above the rotating stone, with corn and grain between the stones.
It is suspected that the force of the water from the cistern was not sufficient to start the wheel turning with the required force, and that a leather thong was wrapped around the shaft and pulled by Indians to start the wheel.
The upper-most room, used now as an art gallery, was used for storage of the final product.
After the water flowed out of the mill, it was channeled into a lake down hill from the mill.
The lake has since dried up and is now the site of San Marino's Lacy Park.

It was the first water-powered grist mill in Southern California, and some have called it the first grist mill in California.
While some have called Father Zalvidea's horizontal design a "mechanical marvel, evolved and constructed by a mastermind," others considered the design flawed as it splashed moisture up the shaft, leaving the flour damp.
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 (Population of Native California) in the mission community.
In 1823, a New England-style mill with a vertical waterwheel was built adjacent to the mission.
The new mill resulted in a superior product, and the old mill ceased operation.
Though there are varying accounts of the exact date, San Gabriel Mission records indicate it was built in 1816. The mill was built on land owned by the San Gabriel Mission, and was designed by Franciscan Father José Maria de Zalvidea, then in charge of the mission. It was built by Tongvan Mission Indian laborers  "under the watchful eye"  of Father Zalvidea.
The mill was built like a fortress. Its lower walls are nearly five feet thick at the base, and are made of brick and volcanic tuff. Some have written that the thick fortress-like walls were intended to allow the padres to barricade themselves in the event of "a disturbance among their somewhat uncertain converts." The upper walls are made of sun-dried adobe slabs, and the building's surface is covered with a lime mortar made from burnt sea shells. The pine and sycamore beams are tied with leather thongs, and the structure is also supported by large buttresses which can still be seen on building's exterior corners.
Water was brought to the mill in an open ditch (zanja) from Los Robles and Kewen Canyons, and stored in a large cistern. For the mill itself, Father Zalvidea designed an unusual horizontal, direct impulse water wheel. There were three vaulted water chambers on the ground floor and a single horizontal water wheel. The water wheel was attached to a vertical shaft that went up to the second level (the current entrance room) where the grinding stones were located. One of the grinding stones was attached to the shaft, and rotated along with the water wheel. A second stone was placed above the rotating stone, with corn and grain between the stones. It is suspected that the force of the water from the cistern was not sufficient to start the wheel turning with the required force, and that a leather thong was wrapped around the shaft and pulled by Indians to start the wheel. The upper-most room, used now as an art gallery, was used for storage of the final product. After the water flowed out of the mill, it was channeled into a lake down hill from the mill. The lake has since dried up and is now the site of San Marino's Lacy Park.
It was the first water-powered grist mill in Southern California, and some have called it the first grist mill in California. While some have called Father Zalvidea's horizontal design a "mechanical marvel, evolved and constructed by a mastermind," others considered the design flawed as it splashed moisture up the shaft, leaving the flour damp. 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 (Population of Native California) in the mission community. In 1823, a New England-style mill with a vertical waterwheel was built adjacent to the mission. The new mill resulted in a superior product, and the old mill ceased operation.
El Molino Viejo