Most of California's Mexican land grants did not survive the American era intact. The Land Act of 1851 required every grantee to prove their claim before a Public Land Commission, a process that took years, consumed legal fees, and often ended in loss through debt or fraud. Rancho Guejito moved through that gauntlet more or less intact. A claim was filed in 1852, and the grant was patented — formally recognized under American law — to George W. Hamley on May 24, 1866.

From there, the land passed through successive owners who expanded it by purchasing adjacent parcels. By the late twentieth century the total acreage had grown from the original 13,299 to approximately 22,359 acres — a working cattle ranch that had kept its essential character across more than a century of ownership changes. In 1974, the state parks division recommended acquiring it as a nature reserve. Governor Jerry Brown vetoed the purchase as too expensive. That same year, industrialist Benjamin Coates bought the land for $10 million, and it remained in the Coates family after his death in 2004.

Between 2003 and 2007, a succession of wildfires burned approximately 93 percent of Rancho Guejito's land. The most significant came in October 2007, when a fire ignited near Guejito Creek in the San Pasqual Valley and merged early on the morning of October 22 with what became known as the Witch Fire. That fire went on to destroy homes in Rancho Bernardo and kill two people. The utilities whose power lines ignited the fire eventually reached settlements with affected landowners.

The ranch's chaparral and grassland burned repeatedly — a pattern familiar throughout the San Diego backcountry, where drought, hot Santa Ana winds, and accumulating fuel loads make fire a near-certainty. What burned can grow back. What is harder to restore is the soil stability and native plant communities that fire disrupts repeatedly over short intervals.

Starting in 2007, as the burned land began to recover, the ranch began planting agriculture. The Rancho Guejito Corporation put in approximately 500 acres of organic avocado groves, citrus trees, and wine grapes — drawing on a deep groundwater supply that the ranch taps from below the valley floor. The operation adopted water-efficient methods including moisture sensors and high-density planting, which allows it to grow crops using roughly one-fourth the water that conventional growers require.

In 2009, a development firm approached San Diego County with a proposal to build approximately 10,000 homes on the property while preserving about 16,000 of the 22,000 acres in a natural state. The proposal drew opposition from conservationists and residents of nearby communities, who argued that the ranch's open character served as a wildlife corridor and buffer for the San Diego Zoo Safari Park. As of 2022, the land remains largely undeveloped, and the ranch launched a direct-to-consumer beef program — its first direct sales in 177 years of continuous ranching operation.

California has hundreds of old rancho names attached to subdivisions, shopping centers, and street signs. Rancho Guejito is different: the land itself has not been broken up. The hills are still hills. The creek still runs where it ran in 1845. The debate over its future — whether it belongs in conservation or development, whether its water should be used for avocados or housing — is ongoing precisely because the land is still whole enough to argue over.

That wholeness is unusual. Most of what Pio Pico granted away in those final years of Mexican California has long since been absorbed into the suburban sprawl of greater San Diego. Rancho Guejito survived partly by luck, partly by geography, and partly because someone kept choosing not to subdivide it. Flying over San Pasqual Valley on a clear day, the distinction is visible: on one side, the rooftops begin; on the other, the chaparral continues unbroken into the hills.