He was a law clerk for United States Supreme Court justices Harlan Fiske Stone (1937-1938) and Stanley Forman Reed (1938).

He was a Chief of Litigation for the Bituminous Coal Division of the United States Department of the Interior from 1939 to 1940.

He then returned to his position as assistant general counsel for the Office of Price Administration in 1946.

He was in private practice of law in Washington, D.C. from 1946 to 1951, before returning to the Office of Price Administration as chief counsel from 1951 to 1952.

[2] Leventhal was nominated by President Lyndon B. Johnson on March 1, 1965, to a seat on the United States Court of Appeals for the District of Columbia Circuit vacated by Judge Wilbur Kingsbury Miller.