Originally home of the Coast Miwok native people, the Mexican government granted Rancho Cotate to Captain Juan Castaneda in July 1844 for his military services in the region. The grant encompassed present-day Penngrove, Cotati and Rohnert Park. "Cotate Rancho is a part of the Vallejo Township which is the plain between Sonoma Mountain and Petaluma Creek San Pablo Bay, and an east and west line dividing the tract from Santa Rosa Township." In 1849, Dr. Thomas S. Page, of Cotati, bought Rancho Cotate. Over time, the property was broken up and sold off piecemeal to incoming settlers.
The town of Rohnert Park was named after the Rohnert family, which owned the Rohnert Seed Farm. In 1929, a successful businessman, Waldo Emerson Rohnert (1869–1933), a native of Detroit, Michigan, purchased a large ranch in the area and minimized flooding in the fields with a crude drainage system. He died shortly thereafter. His son, Fred Rohnert, a graduate of Stanford Law School, took over the ranch and developed a seed growing business, the Rohnert Seed Farm, which became a major horticultural success for the county.
In 1956, only four adults resided within the district boundaries. In 1956 and 1957, with the U.S. Route 101 Freeway recently completed at the Cotati bypass, Rohnert Park began to be laid out and built as a planned city. In a summer election in 1962, Rohnert Park was incorporated, comprising 1,325 acres (536 ha), housing an estimated 2,775 persons. It was the first town to incorporate in Sonoma County since 1905. The neighboring town of Cotati, California, voted to incorporate the following year.