High Quality Content by WIKIPEDIA articles! Imperial County is a county located in the Imperial Valley, in the far southeast of the U.S. state of California, bordering both Arizona and Mexico. It is part of the El Centro Metropolitan Area, which encompasses all of Imperial County. The population as of 2000 was 142,361. The county seat is the city of El Centro. Established in 1907, it was the last county to be established in California. Imperial County is also part of the Southern California border region, also referred to as San Diego-Imperial, the smallest but most economically diverse region in the state.