Miller served during World War I in the 5th Field Artillery Regiment of the American Expeditionary Force, rising through the enlisted ranks (private, corporal, sergeant major) to become an officer (2nd then 1st lieutenant in the 58th Coast Artillery Corps).[7]
From 1928 until 1938, Miller served as chairman of the World Student Christian Federation, and would remain active in his Presbyterian Church throughout his life, as well as publish in Presbyterian Life.[8]
Miller was the organizer and the executive secretary of the National Policy Committee,[9] and helped found the Council on Foreign Relations, whose chief executive he became.[10] During the 1930s, Miller also strongly spoke out against Adolf Hitler and urged war preparedness.[11] He realized that Virginia's U.S. Senator Harry F. Byrd's close financial scrutiny was "nickel and dimming" national war preparedness, because of the Senator's hatred of the administration of Franklin D. Roosevelt, although a fellow Democrat.[12]
While in the Virginia General Assembly (a part-time position) beginning in 1938, Miller allied with Virginia Governor James H. Price, who supported the New Deal despite the opposition of U.S. Senator Harry F. Byrd and his Byrd Organization, which prided itself on low tax rates and cared little about funding human welfare programs. During the Great Depression, Virginia was the 8th lowest in the country in terms of residents on general relief, and ranked 32nd nationally in the amount of state funds obligated for relief.[13] Delegate Miller joined with editors Virginius Dabney and Douglas Southall Freeman, labor commissioner Hall and Virginia Tech's William Garnett to found the Virginia Consumers League. They lobbied for financial security legislation, and succeeded in establishing an Unemployment Compensation Commission in 1938.[14] Miller also helped increase teachers' pay, aid rural libraries and create both a probation and parole system and a merit personnel system for state employees.[8]
Richard R. Farr defeated Miller in 1942, but died before taking his seat. Thus, Robert J. McCandlish Jr. replaced him as Fairfax's delegate in the General Assembly.[15]
During World War II, Miller served for three years (1942–1945) as an intelligence officer with the Office of Strategic Services and the SHAEF. Promoted to the rank of colonel, Miller served on the staff of General Dwight Eisenhower. Miller became the U.S. representative on the Tripartite Control Committee for Operation Sussex.
In 1946, Miller moved his family from the stone farmhouse near Fairfax city where they had lived for decades, to Charlottesville, Virginia, location of the U.S. Army's Officer Candidate School and (beginning in the 1960s) the United States Army Foreign Science and Technology Center. In his 1948 application for membership in the Sons of the American Revolution, Miller characterized his occupation as "MI-Res" and as a writer. He and his wife would also live for a time in Washington, D.C. before their retirement.
After President Harry S. Truman took office upon President Franklin D. Roosevelt's death, and particularly after Truman was elected on his own behalf in 1948 (despite the opposition of the Byrd Organization which was appalled by his support for civil rights legislation), Miller thought he and other "anti's" (including state senators Robert Whitehead and Lloyd M. Robinette) could take over and liberalize the state Democratic party. That never happened, in part because of lack of patronage and financial support from the national Democratic party, in part because the era of Massive Resistance had begun.[16]
In February 1949, Miller announced his candidacy for Governor of Virginia. However, with the assistance of Virginia Beach boss Sidney Kellam (U.S. Senator Byrd technically remaining neutral), John S. Battle defeated Miller and two lesser opponents in the Democratic primary. Battle depicted Miller as a liberal and controlled by labor unions, and nearly ignored his other opponents (Horace Edwards and Petersburg businessman Remmie Arnold). Prominent Republican Henry Wise of Virginia's Eastern Shore even urged his supporters to vote for Battle in the Democratic primary to repel the "invasion by aliens." Battle won 43% of the vote; Miller 35%, Edwards 15% and Arnold 7%.[17]
Undeterred, Miller announced a run against U.S. Senator Byrd himself in 1952, characterizing the powerful senator's record as one of "isolation and indifference." Byrd's allies in the Virginia General Assembly moved the primary from early August to July 15, and Byrd rebuffed Miller's offer to debate, instead delivering over 300 speeches across the Commonwealth (sometimes as many as eight in a day). Byrd handily defeated Miller, winning 63% of the vote.[18] That marked the demise of the "anti's", although their other goals (increased education funding, end of the poll tax, and end of political machine rule) would be achieved a decade later as the Byrd Organization collapsed at the end of Massive Resistance.