Doña Ana County is a county located in the southern part of the New Mexico, United States. As of the 2020 U.S. Census, its population was 219,561, which makes it the second-most populated county in New Mexico. Its county seat is Las Cruces, the second-most populous municipality in New Mexico after Albuquerque, with 111,385 as of th…Doña Ana County is a county located in the southern part of the New Mexico, United States. As of the 2020 U.S. Census, its population was 219,561, which makes it the second-most populated county in New Mexico. Its county seat is Las Cruces, the second-most populous municipality in New Mexico after Albuquerque, with 111,385 as of the 2020 U.S. Census.