Joseph D. Grant, son of a wealthy San Francisco merchant, began buying what had been the Rancho Cañada de Pala Mexican land grant. Grant used the property for grazing cattle, recreating, and hunting game. Joseph D. Grant died in 1942. Santa Clara County purchased approximately 9,553 acres (38.7 km2) of the land in 1975, and created this public park.
Grant Ranch Park is situated in the inner coast ranges near Mount Hamilton. Elevations range from about 1,400 feet (427 m) in Halls Valley to peaks over 2,800 feet (853 m) on the Park's eastern edge. This upward gradient leads to even higher peaks in the crest of the range and to Mount Hamilton itself at 4,209 feet (1,283 m). The western slopes of Mt. Hamilton drop into Smith Creek which forms the eastern boundary of the Park.