Rancho Mirage, also called Mirage, is a city in Riverside County, California, United States. Take a look below for 20 fun and interesting facts about Rancho Mirage, California, United States. 1. The population was 17,218 at the 2010 census, up from 13,249 at the 2000 census, but the seasonal (part-time) population can exceed 20,000. 2….