The Shannon-Williamson Ranch is the only 19th century farm complex in eastern Contra Costa County which has survived intact. It has been owned and operated by descendants of Thomas Shannon for four generations.

The ranch is significant for its association with the early agricultural industry in Contra Costa County, namely wheat and barley, and their importance to the county and state. Wheat was the dominant field crop in the county for nearly half a century until the 1890s. The county was also a leading producer of barley in the state and in the nation.

The buildings have retained their historical integrity, original fabric, and character of the late 1800s. The house, completed around 1896, is an eclectic Italianate style with free classic adaptation and spindlework representative of rural housing built between 1880 and 1900.

Adapted from the NRHP nomination submitted in January 1987.