He worked for the Department of Justice in the 1930s and 1940s, where as director of the alien enemy control unit he oversaw the Japanese American internment.
He contributed to the American Civil Liberties Union's efforts to fight the internment, and after World War II resigned from the Department of Justice to join the ACLU, where he rose up through the organization and eventually served as its president from 1969 to 1976.
Ennis graduated from Columbia University Law School in 1932, and went on to work for the Department of Justice for fourteen years.
Ennis chose not resign from his role at that time, but he did help American Civil Liberties Union head Roger Baldwin to come up with legal strategies to oppose the internment, and testified at the trial of Gordon Hirabayashi about the Department of War's withholding of evidence relating to Japanese Americans' loyalty to the United States.
[1] Ennis died of diabetes complications on January 7, 1990, at Lenox Hill Hospital in New York City.