In 1938, he along with A.C. Buchanan were the choices of Virginia Senators Carter Glass and Harry Byrd Sr. to a vacancy on the United States District Court for the Western District of Virginia, to which Franklin D. Roosevelt named instead Floyd H. Roberts.
[1][2] Following World War II, he was assigned by the Department of the Army to be Counsel under Joseph B. Keenan and later Acting Chief of Counsel of the International Prosecution Section for the International Military Tribunal for the Far East from late 1945 to the end of the trial in 1948.
[1][2] From May 1949 until the mid-1950s, Tavenner served as Chief Counsel for the House Un-American Activities Committee, where he replaced Robert E. Stripling.
"[1][2] Tavenner died age 69 on October 21, 1964, of a heart attack and was buried in the Massanutten Cemetery in Woodstock.
[1][2] Dartmouth College law professor Robert K. Carr wrote of Tavenner that in him HUAC had "obtained perhaps the best qualified and most dispassionate assistant it ever had.