Fremont, California. For the unincorporated community in Yolo County, California, see Fremont, Yolo County, California. Fremont (/ˈfriːmɒnt/ FREE-mont) is a city in Alameda County, California, United States. It was incorporated on January 23, 1956, from the merger of five smaller communities: Centerville, Niles, Irvington, Mission San Jose, and Warm Springs. The city is named after American explorer John Charles Frémont.