In addition to his academic training, Congressman Duncan served in the United States Army National Guard. He enlisted in 1970 and rose to the rank of Captain before completing his service in 1987.

In 1973, Congressman Duncan established a private law practice in Knoxville with Zane Daniel. He was appointed State Trial Judge by Tennessee Governor Lamar Alexander in 1981 and served in that capacity until 1988, when he was elected to Congress.

Congressman Duncan currently serves on the House Transportation and Infrastructure Committee, where he serves as Chairman of the Highways and Transit Subcommittee, and the House Committee on Natural Resources, which retains jurisdiction over national parks and natural resources.

Congressman Duncan's efforts to cut government waste, reduce taxes, and limit bureaucratic red tape have been recognized by various organizations and national news media such as FOX News, ABC News, CBS News, The Wall Street Journal, CNN, U.S. News and World Report, and others. He has been named among the five most fiscally conservative Members of Congress by the National Taxpayers Union and is one of the few Members of Congress to receive the Citizens Against Government Waste Super Hero Award.

In addition to his Washington office , Congressman Duncan maintains offices in Knoxville, Maryville, and Athens. He has established a reputation for providing extensive constituent services and staying in touch with residents of the Second District by attending hundreds of public events and holding countless constituent meetings. Although Congressman Duncan spends more time in East Tennessee than in Washington, he maintains one of the best attendance records in the entire Congress.

Congressman Duncan lives in Knoxville and is an Elder at Eastminster Presbyterian Church. He is married to the former Lynn Hawkins of Sewanee, Tennessee. They have four children (Tara, Whitney, John, and Zane) as well as five grandchildren.