After the internment camps were closed, Opler taught anthropology and sociology at various colleges, including Occidental, Stanford, Harvard, and Tulane from 1946 until 1952. In 1947, Marvin Opler submitted an affidavit in support of the restoration of citizenship to three American citizens of Japanese ancestry who had renounced their citizenship at Tule Lake. In this affidavit, he stated that, rather than being acts of free will, it was coercion, duress, and mass compulsion that motivated many of the wartime renouncements of citizenship by Japanese Americans. At Tule Lake, he had observed many of the renouncement hearings.