The park’s land was once home to many different tribal communities. Over thousands of years, the tribes of the region have come to be collectively recognized as the Ohlone people. Several areas in this location served their dietary and spiritual needs. Ohlone visionaries used the massive rocks to connect to their spiritual world and to acquire personal enlightenment. They also collected a wide variety of medicinal and food plants. 
Native people hunted deer, pronghorns, and bears that were attracted to the area’s abundant vegetation. Today’s park lies within a major trail system that was used to move resources from the coast into the interior.

By 1849, thousands of immigrants had arrived in California searching for gold. The newcomers needed lumber to construct homes and buildings for new towns. The South Pacific Coast Railroad built new lines between the Santa Cruz Mountains and the Alameda port. By 1884, 28 lumber mills operated in the Big Basin/San Lorenzo Valley area; the mills yielded more than 34 million board feet of lumber each year. Coast redwood made excellent lumber, and tanoak bark was used to tan leather goods. 
Agriculture also altered the Castle Rock landscape for nearly 100 years. Farming, livestock, hunting, fishing, and trading made it possible for area families to be self-sufficient. 
The Smead and Partridge farms were the largest operations on the ridge, with orchards of apples, pears, walnuts and grapes. Near the park’s interpretive shelter, heritage trees planted in the early 1900s still bear fruit. 
Judge Joseph Welch
of Santa Clara Valley purchased a 60-acre parcel on Castle Rock Ridge in 1908, while logging was still ravaging the hillsides. He established a precedent by opening his land for the public to enjoy its scenic vistas and rugged landscapes. With Welch’s determination, Castle Rock soon became a popular tourist destination
Dr. Russell Varian, a pioneer of x-ray and radar technology, spent much of his youth exploring and hiking the canyons near Castle Rock. Varian was the first to measure Earth’s magnetic field, using some sites in today’s park. In 1959, he secured an option to purchase this land and planned to donate it to California State Parks. Varian died before completing the purchase. The Sierra Club and Sempervirens Fund later donated the land in his memory. In July of 1968, Castle Rock was designated a state park.