Dean was born in Akron, Ohio and grew up in Marion, the hometown of former President Warren Harding. He attended Colgate University and then The College of Wooster, earning a bachelor's degree in in 1961. He received a Juris Doctor (J.D.) from the Georgetown University Law Center in 1965.

After graduation, he joined a law firm in Washington, D.C.. Having gone to prep school with the son of Barry Goldwater, he was a close friend of the senator and his family. Dean was subsequently employed as the chief minority counsel to the Republican members of the United States House Committee on the Judiciary. A National Commission on the Reform of Federal Criminal Law was created in 1967: Dean was appointed its associate director.

Dean volunteered to write position papers on crime for Nixon's presidential campaign in 1968.The following year he became an Associate Deputy at the office of the Attorney General of the United States in the Nixon administration. In July 1970 he became counsel to the president, after the previous holder of this post John Ehrlichman became the president's chief domestic adviser.

The Start of Watergate

Dean, then White House Counsel, met with Jeb Magruder (Deputy Director of CREEP) and John N. Mitchell (Attorney General of the United States, and Director of CREEP) for a presentation by G. Gordon Liddy on January 27, 1972. At that time, Liddy presented a preliminary plan for intelligence gathering operations during the campaign year 1972. Liddy presented a revised plan to the same group on February 4, which was, however, left unapproved at that stage. A scaled-down plan would be approved by late March of that year. This would lead eventually to attempts to eavesdrop on the Democratic headquarters at the Watergate Complex in Washington, D.C., and to the Watergate scandal.

Linked to cover-up

On February 28, 1973, Acting FBI Director L. Patrick Gray testified before the Senate Judiciary Committee regarding his nomination to replace J. Edgar Hoover as Director of the FBI. Armed with newspaper articles indicating the White House had possession of FBI Watergate files, the committee chairman, Sam Ervin, questioned Gray as to what he knew about the White House obtaining the files. Gray stated he had given reports to Dean, and had discussed the FBI investigation with Dean on many occasions. Gray's nomination failed, and now Dean was directly linked to the Watergate cover-up.

Cooperates with prosecutors

On March 23, the Watergate burglars were sentenced with stiff fines and prison time; Dean hired an attorney and began his cooperation with Watergate investigators on April 6, while continuing to work as Nixon's Chief White House Counsel, never disclosing this obvious conflict to Nixon. On April 22, Nixon requested Dean put together a report with everything he knew about the Watergate matter and even invited him to take a retreat to Camp David to do so.

Fired by Nixon

Coupled with his sense of distance from Nixon's inner circle, "The Berlin Wall" of advisors H.R. Haldeman and John Ehrlichman, Dean sensed he was going to become the Watergate scapegoat, and despite going to Camp David, he returned to Washington without having completed his report. Nixon fired Dean on April 30, the same date he also announced the resignations of Haldeman and Ehrlichman.

Testifies at Senate Committee

On June 25, Dean began his testimony before the Senate Watergate Committee, in which he implicated administration officials, including Nixon fundraiser and former Attorney General John Mitchell, Nixon and himself. He was the first administration official to accuse Nixon of direct involvement with Watergate and the resulting cover-up in press interviews. Such testimony against Nixon, while damaging to the president's credibility, had little impact legally, as it was merely his word against Nixon's. Nixon vigorously denied all accusations against him that he authorized a cover-up, and Dean had no proof beyond various notes he had taken in his meetings with the president. It was not until secret White House tape recordings were made public and analyzed that Dean's accusations were substantiated.

Dean pled guilty to obstruction of justice before Watergate trial judge John Sirica on November 30, 1973. He admitted supervising payments of "hush money" to the Watergate burglars, notably E. Howard Hunt, and revealed the existence of Nixon's enemies list. On August 2, 1974, Sirica handed down a sentence of one to four years in a minimum-security prison. However, when Dean surrendered himself as scheduled on September 3, he was diverted to the custody of U.S. Marshals, and kept instead at Fort Holabird (near Baltimore, Maryland) in a special "safe house" holding facility primarily used for witnesses against the Mafia. He spent his days in the offices of the Watergate Special Prosecutor and testifying in the trial of Watergate conspirators Mitchell, Haldeman, Ehrlichman, Robert Mardian, and Kenneth Parkinson, which concluded on January 1, 1975. Dean's lawyer moved to have his sentence reduced, and on January 8, Sirica granted the motion, adjusting Dean's sentence to time served, which wound up being four months. With his conviction for felony offenses, Dean was disbarred as a lawyer, so could no longer practice law.

Shortly after Watergate, Dean became an investment banker, author, and lecturer.

Dean chronicled his White House experiences, with a focus on Watergate, in the memoirs Blind Ambition (1976) and Lost Honor (1982). Blind Ambition would become the point of controversy for many years after its publication.

In 1992, he hired famed attorney Neil Papiano and brought the first in a series of defamation suits against G. Gordon Liddy for claims in Liddy's book Will, and St. Martin's Press for its publication of the book Silent Coup by Len Colodny and Robert Gettlin. Silent Coup alleged that Dean was the mastermind of the Watergate burglaries and the Watergate coverup, and the true target of the burglaries was to seize information implicating Dean and Maureen Biner (his then-fiancée) in a prostitution ring. After hearing of Colodny's work, Liddy issued a revised paperback version of Will supporting Colodny's theory. This theory was subsequently the subject of an A&E Network Investigative Reports series program entitled The Key to Watergate in 1992. In the preface to his 2006 book, Conservatives Without Conscience, Dean strongly denied Colodny's theory, pointing out that Colodny's chief source (Phillip Mackin Bailley) had been in and out of mental institutions. Dean settled the defamation suit against Colodny and his publisher, St. Martin's Press, on terms which Dean stated in the book's preface he could not divulge under the terms of the settlement, other than stating that "the Deans were satisfied." In the footnote to this portion of the preface, Dean stated that the federal judge handling the case forced a settlement with Liddy.

Dean retired from investment banking in 2000 while continuing to work as an author and lecturer, becoming a columnist for FindLaw's Writ online magazine. He currently resides in Beverly Hills, California.

In 2001, Dean published The Rehnquist Choice, an exposé of the White House's selection process for a new Supreme Court justice in 1971, which led to the accession of William Rehnquist to the United States' highest court. Three years later, Dean authored a book heavily critical of the administration of George W. Bush, entitled Worse than Watergate, which called for the impeachment of Bush and Vice-President Dick Cheney for allegedly lying to the Congress.

His subsequent book, released in summer 2006, is titled Conservatives without Conscience, a play on Barry Goldwater's The Conscience of a Conservative. In it, he asserts that post-Barry Goldwater conservatism has been co-opted by people with authoritarian personalities and policies (citing data from Robert Altemeyer). According to Dean, modern conservatism, specifically in the Christian Right, embraces obedience, inequality, intolerance, and strong intrusive government, in stark contrast to Goldwater's philosophies and policies. Using Altemeyer's scholarly work, he contends that there is a tendency toward ethically questionable political practices when authoritarians are placed in positions of power, and that the current political situation is dangerously unsound because of it. Dean cites the behavior of key members of the Republican leadership — including George W. Bush, Dick Cheney, Tom DeLay, Newt Gingrich and Bill Frist — as clear evidence of a relationship between modern right-wing conservativism and this authoritarian approach to governance. He places particular emphasis on the abdication of checks and balances by the Republican Congress, and of the dishonesty of the conservative intellectual class in support of the GOP, as a result of the obedience and arrogance innate to the authoritarian mentality.

After it became known that George W. Bush authorized NSA wiretaps without warrants, Dean asserted that Bush is "the first President to admit to an impeachable offense". On March 31, 2006, Dean testified before the Senate Judiciary Committee during hearings on censuring the president over the issue. Sen. Russell Feingold (D-Wis.), who sponsored the censure resolution, introduced Dean as a "patriot" who put "rule of law above the interests of the president." In his testimony, Dean asserted that Richard Nixon covered up Watergate because he believed it was in the interest of national security. This sparked a sharp debate with Republican South Carolina senator Lindsey Graham, who repeatedly asserted that Nixon authorized the break-in at Democratic headquarters. Dean finally replied, "You're showing you don't know that subject very well." According to Washington Post reporter Dana Milbank, "Spectators laughed, and soon the senator was sputtering mad."

Dean's 2007 book Broken Government: How Republican Rule Destroyed the Legislative, Executive and Judicial Branches is, as he wrote in its introduction, the third volume of an unplanned trilogy. In this latest book, Dean, who has repeatedly described himself as a Goldwater conservative, built on Worse Than Watergate and Conservatives Without Conscience to argue that the Republican Party has gravely damaged all three of the branches of the federal government in the service of ideological rigidity and with no attention to the public interest or the general good. Dean concludes that conservatism must regenerate itself to remain true to its core ideals of limited government and the rule of law.

In 2008, Dean co-edited with Barry M. Goldwater, Jr., Pure Goldwater, a collection of writings by the 1964 Republican presidential nominee and former U.S. Senator from Arizona Barry Goldwater, in part as an act of fealty to the man who defined his political ideals.

In the 1979 TV mini-series, Blind Ambition, Dean was played by Martin Sheen. In the 1995 film, Nixon, Dean was played by David Hyde Pierce. In the 1999 film Dick, Dean was played by Jim Breuer.

Dean frequently serves as a guest on the MSNBC show, Countdown with Keith Olbermann, and the Randi Rhodes Show on the Nova M Radio network.

In January, 2009, a new controversy arose about Dean. Historian Stanley Kutler was accused of editing the Nixon tapes to make Dean appear in a more favorable light.

On September 17, 2009, Dean appeared on Countdown, with new allegations about Watergate in hand. He stated that he had found information via the Nixon tapes, that showed what the burglars were after: information on a kickback scheme involving the Democratic National Convention in Miami, Florida. Dean also asserts that Nixon did not directly order the break in, but that it was ordered by Ehrlichman on behalf of Nixon.

He appeared on Chicago Tonight on October 21, 2009 to comment on the book "The Harding Affair"