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. As required by the Land Act of 1851, a claim for Rancho Atascadero was filed with the Public Land Commission in 1852, and the grant was patented to Henry Haight in 1860.
The rancho was sold in 1864 to Martin Murphy Jr. (1807–1884) and his wife Mary Bulger Murphy (d.1892) of Sunnyvale, who had come to California with the Stephens-Townsend-Murphy Party in 1844. The Murphys turned over running of the rancho to their son Patrick Murphy, who was a General in the California National Guard.  Patrick Washington Murphy (1840–1901) operated Rancho Atascadero, and the adjacent Rancho Asuncion, and Rancho Santa Margarita, altogether comprising about 61,000 acres (247 km2), from his Rancho Santa Margarita headquarters.
In 1912, Edward Gardner Lewis, a successful magazine publisher, bought Rancho Atascadero from J.H. Henry. Lewis founded the utopian, planned community of Atascadero in 1913.
- Ogden Hoffman, 1862, Reports of Land Cases Determined in the United States District Court for the Northern District of California, Numa Hubert, San Francisco
- Diseño del Rancho Atascadero
- U.S. Geological Survey Geographic Names Information System: Rancho Atascadero
- Hoover, Mildred B.; Rensch, Hero; Rensch, Ethel; Abeloe, William N. (1966). Historic Spots in California. Stanford University Press. ISBN 978-0-8047-4482-9.
- United States. District Court (California : Southern District) Land Case 113 SD
- Report of the Surveyor General 1844 - 1886
- J. P. Munro-Fraser, 1881, History of Santa Clara County, California, Alley, Bowen & Co, San Francisco
- Biography of Martin Murphy, Jr.
- Thompson & West , 1883, History of San Luis Obispo County California, Oakland
- Patrick Washington Murphy