During World War II, Du Val served in the U.S. Navy (1942–1946), including in the Pacific Theater aboard the USS Lexington, an aircraft carrier.

In 1959, Du Val served as Associate General Counsel for the President's Committee to Study the United States Military Assistance Program.

Du Val returned to private practice in 1959, with the Washington D.C. office of the New York law firm Milbank, Tweed, Hadley & McCloy, where he worked in legislative affairs until retiring in 1970.

Du Val became President of the McLean Citizens Association and succeeded in getting the U.S. Department of the Interior to buy a conservation easement and thus block a proposed high rise development along that segment of the Potomac River.

The following year Du Val was one of only two northern Virginia Democratic delegates winning re-electing in what became a Republican landslide after an address by President Richard Nixon (and the collapse of the Byrd Organization).

A consumer advocate, DuVal challenged electric and telephone utility rate increases before the State Corporation Commission and Virginia Supreme Court.

He also worked with the Virginia Citizens Consumer Council, Izaak Walton League, Sierra Club, National Trust for Historic Preservation, and with veterans organizations and Chambers of Commerce in his district.

Despite traveling to Richmond on a Greyhound bus and staying at an inexpensive motel, Du Val had risen to lead the Democratic Caucus.