Rancho San Luis Gonzaga was a 48,821-acre (197.57 km2) Mexican land grant in the Diablo Range, in present-day Santa Clara County and Merced County, California given in 1843 by Governor Manuel Micheltorena to Juan Perez Pacheco and José Maria Mejía. The grant was bounded by Francisco Pacheco's Rancho Ausaymas y San Felipe on the west (at the top of present-day Pacheco Pass), the San Joaquin River and San Joaquin Valley on the east, and Los Baños Creek on the south.
A grant was first made in 1841 to Francisco Jose Rivera of Monterey, but he returned to Mexico soon after and did not occupy the grant. The eleven square league grant was made to Juan Perez Pacheco and José Maria Mejía in 1843. Three days later, Captain Mejia gave his half of the grant to Pacheco. Juan Perez Pacheco (1823–1855) was the son of Francisco Perez Pacheco (1790–1860), grantee of Rancho Ausaymas y San Felipe. The rancho lay at a great crossroad where the road from Pacheco Pass into the San Joaquin Valley crossed the El Camino Viejo that lay along the west side of the valley. Its lands included the land and adobe ranch house of the old Spanish Rancho de Centinela (Sentinel Ranch) first established by pioneering stockmen from San Juan Bautista and Monterey as place to raise horses in 1810 and subsequently abandoned in the 1820s.
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 San Luis Gonzaga was filed with the Public Land Commission in 1852, and the grant was patented to Juan Perez Pacheco in 1871.
When Juan Perez Pacheco died in 1855, the property went to his father, Francisco Pacheco. In 1858, the rancho became a stage station for the Butterfield Overland Mail.
Upon Francisco Pacheco's death in 1860, his only surviving child, Ysidora Pacheco (1829–1892) inherited most of the Pacheco holdings. In 1850, Ysidora married Mariano Malarin (1827–1895) of Rancho Chualar. When María Ysidora Pacheco died in 1892, her estate consisted of Rancho San Luis Gonzaga and half of Rancho Ausaymas y San Felipe.
Paula Fatjo, a great granddaughter of Ysidora and Mariano Malarin, inherited 16,000 acres (64.7 km2) of the ranch land in 1948, and used it to raise horses and cattle. The majority of her property was condemned by the state of California in 1962 to create the San Luis Reservoir, and the original 1846 ranch house, which she had restored, was destroyed in an attempt to move it away from the area flooded by the new lake. Fatjo died on December 30, 1992, leaving the remaining 6,890 acres (27.9 km2) to the California Parks System, where it forms what is now Pacheco State Park.
- Ogden Hoffman, 1862, Reports of Land Cases Determined in the United States District Court for the Northern District of California, Numa Hubert, San Francisco
- Early Santa Clara Ranchos, Grants, Patents and Maps
- Marjorie Pierce,1981, East of the Gabilans, Western Tanager Press, ISBN 978-0-934136-11-2
- 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 : Northern District) Land Case 62 ND
- Report of the Surveyor General 1844 - 1886 Archived 2009-05-04 at the Wayback Machine
- Portraits of Isidora Pacheco and Mariano Malarín by Leonardo Barbieri
- Will of Isadora Pacheco Malarin, Salinas Weekly Index, Thursday, Aug. 25, 1892
- Rodebaugh, Dale (May 24, 1996), "Heiress' ancestral landholds become a Northern California state park", San Jose Mercury-News, archived from the original on March 29, 2015.