From 1943 until 1946, Opler worked as a Community Analyst at the Tule Lake Japanese Internment Camp, where his critical views of the internment of Japanese Americans later led him to co-author "Impounded Peoples" in 1946. While at Tule Lake, he kept careful records of daily camp life. Opler documented instances of abuse at the camp and worked with lawyer Wayne M. Collins on behalf of the internees. His records included an account of "The November Incident," a protest by the residents of the camp which resulted in the takeover of Tule Lake by the US Army. Author Barney Shallit remembered Marvin Opler at Tule Lake both fondly and vividly: "with his heavy red beard and his slow, deliberate movements, he looked ... like a benign, giant panda." Marvin's wife, Charlotte Opler, enrolled their son Ricky in the Japanese nursery camp at the center, making him the only Caucasian enrolled there. Marvin Opler noted the parallels between the revival of traditional Japanese culture among the largely acculturated internees at Tule Lake and the spread of the Ghost Dance religion among Plains Indian tribes in the 19th century. Opler pointed out that both were attempts by the colonized to reassert their dignity.