In 2000, the Castle Rock Ranch Acquisition Act was passed, and money from the Conservation and Access funds were used to purchase the private ranch at Castle Rocks. Following the purchase by the National Park Service, the lands were transferred to the state of Idaho:
After the acquisition, the NPS exchanged the property with the Idaho Department of Parks and Recreation (IDPR) for other lands adjacent to existing NPS properties. Since May 25, 2003, the IDPR has provided park facilities and managed recreation at Castle Rocks.[2]
The park was expanded by 200 acres (81 ha) in 2007.