Completed about 1816 as a grist mill for Mission San Gabriel, the original two-story structure measured 53 by 26 feet. El Molino Viejo is the oldest commercial building in Southern California.
- The lower walls are five feet thick and are composed of oven-baked brick and volcanic tuff, while the walls of the upper level are built of layers of sun-dried adobe slabs. Rafters, ceiling, and beams are made of local pine and sycamore; the roof is tiled. The whole surface of the building is covered with mortar made from lime derived from burnt sea-shells, and buttresses supporting three corners supply additional strength. These were needed not only as reinforcement against earthquakes, but also to counteract the vibration of the machinery.
- The present entrance room (with its collection of antique paintings and furnishings) served as the Grinding Room of the mill. Here the heavy millstones ground the grain harvested on the mission lands. A set of millstones, discovered by General George Patton during his childhood in this area, has been preserved on the patio.
- The upper room, on the top level, was originally used as the Granary, where the milled grain was stored. The California Art Club maintains a gallery where outstanding paintings are displayed for sale. The exhibitions change every three months.
- On the western side of the building can still be seen the structural outlines of a water tank. This provided the energy to operate the machinery. The water flowed through the mill eastward through a ditch to Mission Lake, dammed by the padres. The lake area is now known as San Marino’s Lacy Park.
- The small room on the west side of the building next to the cisterns was formed from part of the old cistern and serves as the Foundation’s office.
- Downstairs there is a small diorama display featuring an operating model of the mill as it was in those early days. This photographic display chronicles the life of the Mill since 1816, and the families who have called it home.
- Outside of The Mill is an attractive, well-maintained garden highlighting native California trees and plants. The centerpiece of the garden is the Pomegranate Patio where many civic, educational, and organizational events are held during the spring and summer months. This patio is the home of the Magical Music At The Mill chamber music concerts each summer.