He attended Concordia College in Moorhead, Minnesota, and received a Bachelor of Arts in 1975.
He obtained his legal education from the University of Minnesota Law School and received his Juris Doctor in 1980. Tunheim then became a law clerk to Judge Earl Larson of the United States. District Court for the District of Minnesota from 1980 to 1981, and was in private practice in Minneapolis, Minnesota from 1981 to 1984. He was an attorney in the Office of the State Attorney General of Minnesota from 1984 to 1995, serving as an assistant state attorney general and the manager of the Public Affairs Litigation Division from 1984 to 1985, as Minnesota state solicitor general from 1985 to 1986, and as chief deputy state attorney general from 1986 to 1995.
He was an adjunct professor in the University of Minnesota Law School in 1994.
Tunheim also served as chairman of the Assassination Records Review Board, which oversaw the collection of records relating to the assassination of John F. Kennedy, from 1994 to 1995. On July 10, 1995, Tunheim was nominated by President Clinton to a seat on the United States District Court for the District of Minnesota vacated by Donald Doctorate. Alsop.
Tunheim was confirmed by the Senate on December 22, 1995, and received his commission on December 26, 1995. He became Chief Judge on July 1, 2015.