Prior to European colonization, the range was a seasonal village and hunting site for the
Hohokam and later the
Tohono O'Odham and
Hia-Ced O'Odham peoples. The first European known to have visited the area, Italian Jesuit
Eusebio Kino, established a mission at the O'Odham village of Son Oidag in the valley below the range in 1701, which grew into the Mexican town of
Sonoyta, Sonora in the 1850s. Following the
Gadsden Purchase of 1854 the US/Mexico border was redrawn through the Ajo Mountains, and American miners and ranchers began to settle nearby. Most notable among them was the Gray family, who maintained line camps and pastures throughout the range at sites including Alamo Canyon and Bull Pasture. The eastern half of the range became part of the
Tohono O'Odham Reservation in 1916, while on the western half most human uses, both indigenous and settler, were forcibly ended following the range's designation as part of
Organ Pipe Cactus National Monument in 1937. The Gray family, however, was allowed to continue grazing in the monument, although this did not prevent a decades-long legal battle from erupting between the Grays and the federal government, as the Park Service argued Gray cattle were damaging the ecology of the monument while the Grays were unwilling to give up their traditional way of life. While the case was never resolved in court, the last Gray heir died in 1976 and no grazing has been permitted in the Ajo Range, or anywhere else on Organ Pipe, since. Given their location on the US/Mexico border, the Ajo Mountains have long been a corridor for travel between the two countries. This traffic was largely unregulated for Mexican and American citizens in the 19th and early 20th centuries, although the passage of the
Chinese Exclusion Act in 1882 forced Chinese immigrants to enter clandestinely. Many Chinese immigrants are known to have crossed in or around the Ajo Range. The first US Border Patrol agent,
Jeff Milton, worked the Sonoyta corridor, including the Ajo Range, with the job title of "Mounted Chinese Inspector" in the early 20th century. Following the implementation of
Prevention Through Deterrence in the 1990s the range has become a major corridor for clandestine traffic from Mexico, and is heavily traveled by immigrants as well as
Border Patrol and humanitarian groups such as
No More Deaths. == Ecology ==