Tanner was born in Halls, Tennessee and he graduated from the University of Tennessee. Tanner served in the United States Navy from 1968 to 1972. He was elected to the Tennessee House of Representatives as a Democrat in 1976, replacing Larry Bates, who mounted an unsuccessful primary challenge against Congressman Ed Jones. When Jones retired in 1988, Tanner won the Democratic nomination for the seat and handily defeated Republican nominee Ed Bryant, who went on to represent the neighboring 7th District from 1995 to 2003.

Tanner was reelected in 1990 with no major-party opposition, a feat he repeated in 1992. In 1998, he was completely unopposed. He handily defeated Republican opponents in 1994 (the only time besides his initial election that he faced a serious or well-funded Republican), 1996, 2000, 2002 and 2004. His 2004 opponent, James L. Hart, was an avowed eugenicist disavowed by the state Republican Party.

As a Congressman, Tanner has sponsored a bill to repeal the inheritance tax (which was vetoed by President Clinton) and he is in favor of a balanced budget. It is reported that Tanner could have been appointed to the United States Senate by governor of Tennessee Ned McWherter in 1992 to replace Al Gore but he declined the offer, and Harlan Mathews was appointed as a caretaker instead. Tanner became nationally known briefly when it was alleged that President Clinton was on the telephone with him in 1995 during one of Clinton's sexual encounters with Monica Lewinsky. Tanner was a founder of the Blue Dog Democrats and has denied rumors that he might switch parties, and has an earned reputation as a moderate.

Tanner is strongly in favor of balancing the budget and paying down the national debt, and has been a strong opponent of the fiscal policies of President George W. Bush, voting against virtually all tax cuts passed since his taking office. Tanner was one of the few Democrats in the House to vote in favor of CAFTA and has long distanced himself from the majority of his party on issues such as bankruptcy law and lawsuit reform. He voted for the Federal Marriage Amendment, the ban on "partial-birth" abortions, limiting death penalty appeals, and has voted against most gun control measures. On other issues he is more liberal: he often votes with his party on separation of church and states issues, and has consistently voted against the Flag Desecration Amendment. Tanner voted with the majority of his party to expand stem cell research and against renewing the controversial portions of the Patriot Act. He also supports affirmative action and public education. Tanner was firmly opposed to Bush's attempt to reform Social Security.

Tanner received much of his knowledge of politics as a youth from his mother Doris, who was an associate professor of history at the University of Tennessee-Martin and a former Women's Auxiliary Service Pilot (WASP) in World War II who had long been a leading advocate for these women to receive veteran status, which they were eventually granted.

In 2004, Congressman Tanner made a brief cameo appearance alongside Michael Moore in Fahrenheit 9/11.

He recently drafted a bill that would forbid state legislatures from drawing congressional districts. It is generally believed that this bill is a response to Republican-inspired mid-decade redistricting in Texas and recent similar efforts in Colorado and Georgia.