He was elected on October 18, 1937, as a Democrat to the United States Senate to fill the vacancy caused by the death of United States Senator Joseph Taylor Robinson for the term ending January 3, 1943, and served from November 15, 1937, until his resignation effective March 31, 1941, to assume a federal judicial post.

[1] Miller was nominated by President Franklin D. Roosevelt on January 31, 1941, to a seat on the United States District Court for the Western District of Arkansas vacated by Judge Heartsill Ragon.

He was confirmed by the United States Senate on January 31, 1941, and received his commission on March 12, 1941.

He served as Chief Judge from 1958 to 1967 and as a member of the Judicial Conference of the United States from 1962 to 1963.

[2] At the time of his death, he resided in Fort Smith, Sebastian County, Arkansas.