Ajo is an unincorporated community and a census-designated place in Pima County, Arizona, United States. The population was 3,705 at the 2000 census. Ajo is located on Arizona Highway 85 just 43 miles from the Mexican border. It is the closest town to Organ Pipe Cactus National Monument. Ajo apartments for rent comparison page shows most up to date rent information on Ajo apartments and houses. Rent information is obtained from multiple rental sources but we cannot guarantee their accuracy.