Mesa is a city in Maricopa County, in the U.S. state of Arizona. It is the most populous city in the East Valley section of the Phoenix Metropolitan Area. It is bordered by Tempe on the west, the Salt Rive…Mesa is a city in Maricopa County, in the U.S. state of Arizona. It is the most populous city in the East Valley section of the Phoenix Metropolitan Area. It is bordered by Tempe on the west, the Salt River Pima-Maricopa Indian Community on the north, Chandler and Gilbert on the south along with Queen Creek, and Apache Junction on the east.