Born in Regent, North Dakota, Dorgan earned a B.S. from the University of North Dakota in 1964 and an M.B.A. from the University of Denver in 1966. He served as tax commissioner of North Dakota from 1969 until 1980, when he was elected to the United States House of Representatives, of which he was a member from 1981 until 1992, being reelected every two years.

In 1992 he ran for the Senate when the Democratic incumbent, Kent Conrad, did not run. Dorgan won the election. He took office a few weeks early, in December 1992, when the governor appointed him to fill the rest of the term of Conrad, who had won a special election to fill the rest of the term of deceased senator Quentin N. Burdick. Dorgan was reelected in 1998 and 2004.

As chairman of the Democratic Policy Committee, Dorgan is one of the most powerful Democrats in the Senate. In recent years he has been increasingly sought by the national media for comment on political issues. He is a strong opponent of the U.S. policy towards Cuba. He has introduced, with varying levels of success, several amendments to end the prohibition on travel to Cuba and to terminate funds for anti-Castro broadcasting. Dorgan has also opposed most bills liberalizing trade policies between the United States and other countries. He has a mixed record on lawsuit reform, voting against the Private Securities Litigation Reform Act and the Class Action Lawsuit Fairness Act of 2005, but voting in favor of the failed Common Sense Product Liability and Legal Reform Act and legislation to shield gun manufacturers and distributors from lawsuits. He is briefly featured in Michael Moore's documentary Fahrenheit 9/11, in which he discusses the aftermath of the September 11, 2001 attacks. He was not being interviewed by Moore.