Rancho Mission Viejo is an active 23,000 acres ranch and farm, habitat reserve, residential community, and census-designated place in South Orange County, California. Rancho Mission Viejo originated as a series of land grants to John Forster in 1845. The remaining part of Rancho Mission Viejo consists of a nearly 17,000-acre nature reserve and multiple re…Rancho Mission Viejo is an active 23,000 acres ranch and farm, habitat reserve, residential community, and census-designated place in South Orange County, California. Rancho Mission Viejo originated as a series of land grants to John Forster in 1845. The remaining part of Rancho Mission Viejo consists of a nearly 17,000-acre nature reserve and multiple residential communities slated to open in phases between 2010 and 2030. As of the 2020 census, it had a population of 10,378.