The structure at 9050 Memory Lane was built in 1856 by Augustus Ensworth, making it the oldest surviving Anglo-American building in Spring Valley. Ensworth put it up nine years after California became a U.S. territory and four years before the Civil War. In 1865 it passed to Rufus King Porter, son of the founder of Scientific American, who used it as a working farm. Porter sold it to Bancroft in 1885, along with roughly 500 acres of surrounding land. By then Bancroft had already begun — and largely completed — the great historical project that would consume two decades of his life. The ranch was where he could think, rest, and work away from his San Francisco publishing operation.

A spring on the Bancroft property had been known for generations as El aguaje de San Jorge — the watering place of Saint George. This spring gave Spring Valley its name, which puts the naming of the entire community on the same land where Bancroft's ranch house stands. The spring was not merely decorative; in a dry county where water determined what was possible, a reliable spring was the reason land was settled at all. Bancroft recognized what he had: a well-watered property in a region where water was never taken for granted, close enough to San Diego for convenience but rural enough for the sustained concentration that historical writing requires.

Hubert Howe Bancroft was a San Francisco bookseller who became the most ambitious historian of the American West anyone had yet attempted to be. His 39-volume Works of Hubert Howe Bancroft, published between 1882 and 1890, covered the native peoples, colonization, and political history of the entire western region from Alaska to the Isthmus of Panama. The scale required a library, and Bancroft built one — acquiring more than 60,000 volumes of books, manuscripts, maps, and documents. After his death, the collection was purchased by the University of California and became the Bancroft Library at Berkeley, which remains one of the premier research collections for western American history. The man who assembled it spent at least some of his working life on the Spring Valley ranch.

The ranch house was designated a National Historic Landmark in 1962, a recognition grounded in Bancroft's significance as a historical figure rather than in any particular architectural distinction. The building is a vernacular adobe and wood structure from 1856, solid but not ornate — the kind of thing a practical settler built to last rather than to impress. It has lasted. The designation gives it formal protection and situates it within the broader story of California's pre-statehood and early American period. For Spring Valley, it anchors a local history that predates the suburban development surrounding it by more than a century — the oldest structure in the community, still standing on the land where the spring runs.