Monterrey is the capital and largest city of the northeastern state of Nuevo León, Mexico, and the ninth largest city and second largest metro are…Monterrey is the capital and largest city of the northeastern state of Nuevo León, Mexico, and the ninth largest city and second largest metro area in Mexico behind Greater Mexico City. Located at the foothills of the Sierra Madre Oriental, Monterrey is a major North American business and industrial hub, as well as the most populous city in Northern Mexico.