Opler's chief anthropological contribution was in the ethnography of Southern Athabaskan peoples, i.e. the Navajo and Apache, such as the Chiricahua, Mescalero, Lipan, and Jicarilla.

[2] He also aided the defense of Gordon Hirabayashi and Fred Korematsu in their unsuccessful cases challenging the legality of the exclusion of Japanese Americans from the West Coast.

Opler wrote an amicus brief for each case that argued the military necessity cited by Western Defense Command head John L. DeWitt was in fact, based "on racial grounds.

[3] Morris Edward Opler was not the first to anthropologically study and work with the Apache people, nor was he the sole voice contributing to their historical narrative.

This research inspired his dissertation, entitled "An Analysis of Mescalero and Chiricahua Apache Social Organization in the Light of Their Systems of Relationships," which he presented in 1932.

[5][6] Ten years later, in 1942, while Opler was working at Claremont College, he was awarded a Fellowship grant from the John Simon Guggenheim Memorial Foundation as a result of his research among the Apache people during the decade prior.

[5] In part because of Opler's work, the Supreme Court ruled in 1945 that the Japanese internees were being held and treated unconstitutionally, and after that, they were filtered back into everyday society.

[8] In 1949, after he had completed his research and work at Manzanar in California, Opler returned to New York and accepted a position at Cornell University.

He had a lifelong interest in the indigenous people of western America, specifically the Apache, and consistently focused his studies on their lifestyles and practices.

[6] In addition to his anthropological studies, Opler entered the world of academia, working as a professor for many years, beginning in 1937, when he was employed at Reed College.

He lived in a time when the United States was experiencing widespread paranoia surrounding Marxism, and anthropologists were often the group found most guilty of engaging in Marxist mindsets and practices.

He believed that the unchanging nature of human biology and the constant evolution of culture would contradict each other if attempts were made to study them in tandem.

This racial tolerance would lead him to dedicate so much of his research efforts to marginalized ethnic groups, namely the Native Americans and the Japanese.