With the cession of California to the United States following the Mexican-American War, the 1848 Treaty of Guadalupe Hidalgo provided that the land grants would be honored.

[9] In 1864, Forster added Rancho Santa Margarita y Las Flores to his holdings, which then totaled about 200,000 acres (81,000 ha), making him one of the largest landowners in the state.

Flood provided the money to purchase the ranches; O'Neill, offering his skills as a cattleman as sweat equity, agreed to work out his half as resident manager.

Under O'Neill, the cattle herd was expanded, the land was improved, row crops were introduced, and the ranch became Orange County's largest producer of wheat.

In 1942, the United States Marine Corps acquired the entire San Diego portion of 123,620 acres (500.3 km2) for $4,239,062 to expand Camp Pendleton.

After the war, what remained of the historic Ranch now encompassed two Orange County parcels, united under the name Rancho Mission Viejo, and totaling 52,000 acres (210 km2).

The three brothers and their uncle, Richard O'Neill, have preserved more than 20,000 acres (81 km2) of open space and moved forward with additional development of the former Rancho.

Rancho Mission Viejo is still a working ranch with 600 head of cattle and has more than 500 acres (2.0 km2) of citrus trees, as well as crops of avocados, beans and barley.

In 2000, the O’Neill/Avery/Moiso family created a comprehensive open space preservation and land use plan for the remaining 23,000 acres of Rancho Mission Viejo.

Cow Camp Road will eventually connect to Ortega Highway in the east, providing a four mile route along the area, with new housing developments being built between 2010 and 2030.