Posner was born in San Francisco, the only child of Jerry and Gloria Posner, native San Franciscans. His father was a labor union official.

Posner was educated at St. Ignatius College Preparatory (1972), the University of California, Berkeley (1975), and Hastings Law School (1978). He worked for prominent law firm Cravath, Swaine & Moore until 1980 when he went into private practice with a partner. John Martin of ABC News says "Gerald Posner is one of the most resourceful investigators I have encountered in thirty years of journalism." Garry Wills calls Posner "a superb investigative reporter," while John Balzar, reviewing one of his books in the Los Angeles Times, dubs him "a classic-style investigative journalist." Richard Bernstein, reviewing one of his books in the New York Times, lauded his "exhaustive research techniques".

When Posner was hired at Cravath at age 23, he was one of the youngest attorneys ever hired by them. A Phi Beta Kappa and Summa Cum Laude graduate of the University of California at Berkeley (1975), he was an Honors Graduate of Hastings Law School (1978), where he served as the Associate Executive Editor for the Law Review. He left the law in 1986, when his first book, about Nazi Dr. Josef Mengele's life on the run, was published by McGraw Hill.

He is married to author and journalist Trisha Posner.

Case Closed

In his book Case Closed, Posner contended that Lee Harvey Oswald acted alone in the assassination of John F. Kennedy and Oswald's assailant, Jack Ruby, acted independently as well. Case Closed was a finalist for the 1994 Pulitzer Prize for History. It was also the subject of a double issue of U.S. News and World Report, and featured on programs such as ABC's 20/20, CBS Special Reports, and PBS's Frontline. The book has been optioned for a television miniseries.

Posner testified before Congress about the findings in Case Closed. However, the veracity of elements of his testimony has been questioned. In particular, certain individuals that Posner claimed to have interviewed (James Tague, a witness lightly wounded in the assassination, and J. Thorton Boswell, one of the autopsy pathologists) said that they had never spoken to Posner, and both autopsy pathologists denied positions that Posner attributed to them. Posner claimed that he had tapes of himself interviewing Tague and Boswell. Posner also cited his phone bill as evidence of the conversations. However, he made the bill available on only a restricted basis, and discrepancies in the bill have resulted in questions about its validity. Posner never replied to repeated requests from the governmental Assassination Records Review Board for his interview materials, despite initial promises during his Congressional testimony that he would make these available.

Case Closed drew widespread criticism from assassination researchers who contended that it contained factual inaccuracies. For example, historian David Wrone wrote that "massive numbers of factual errors suffuse the book". Vincent Bugliosi, whose own book Reclaiming History largely agrees with Posner's conclusions, accused Posner of "omissions and distortions" but also described Case Closed as "an impressive work".

Secrets of the Kingdom

In his 2005 book Secrets of the Kingdom: The Inside Story of the Secret Saudi-U.S. Connection, Posner provides an account of the "close" business and personal relationship between the House of Saud and the U.S. government, including discussions of "dirty bomb" technology and the financial and political maneuvering surrounding 9/11. Posner also asserts that the Saudis have built an elaborate doomsday scenario around their oil fields. The Saudis have denied this, and some skepticism has been expressed about the plausibility of Posner's account of such a scheme. According to Posner, he and his wife, Trisha, have been banned from entering Saudi Arabia as a result of this book.

Why America Slept

Another 2003 book by Posner, Why America Slept, discusses the conspiracy of the Arab al-Qaeda terrorists who were responsible for the September 11, 2001 attacks. In the book Posner claims that Prince Ahmed bin Salman bin Abdulaziz Al Saud had ties to al-Qaeda and advance knowledge of the 9/11 attacks. This assertion was strongly denied by Prince Ahmed's family, who pointed out that he in fact loved America, spent time at his home there, and invested heavily in the American horse racing industry. Prince Ahmed, two other Saudi princes named by Posner, and the chief of the Pakistani Air Force, all died within days of each other, either from a blood clot after a simple operation, a car wreck involving only one vehicle, dehydration in the desert, or a sabotaged helicopter explosion. Three of the men were in their forties, and one in his twenties. In Why America Slept, Posner became the first journalist to reveal the details of an American interrogation against one of the highest ranking Al Qaeda suspects caught to date. Why America Slept reached #2 on the New York Times best-seller list.

Miami Babylon

This 2009 book explores the history of Miami Beach, with a particular focus on corruption, extravagance, and the drug trade. Some of the individuals interviewed by Posner for Miami Babylon have complained of severe misquoting and inaccuracies. Miami Babylon has been optioned for a television series.

Editorial writings

Posner was the Chief Investigative Reporter for The Daily Beast, until his dismissal due to plagiarism. He was a strong supporter of Al Gore for the 2000 presidential election, and wrote a Wall Street Journal editorial shortly after the 9/11 attacks reversing his opinion of George W. Bush. Later he changed his opinion again; in October, 2006, in " An Open Letter to the President," published on The Huffington Post, he reverted to his original position that Bush was a bad president stifled by his stubbornness. He has also written about investigative issues for The New York Times, The New Yorker, Vanity Fair, Talk, Newsweek, Time, the Miami Herald, and the London Telegraph. He is a regular contributor to NBC's Today Show, as well as other national shows on the History Channel, CNN, FOX News, and CBS. He is a frequent guest on MSNBC's Countdown with Keith Olbermann. A member of the National Advisory Board of the National Writers Union, Posner is also a member of the Authors Guild, International PEN, The Committee to Protect Journalists, and Phi Beta Kappa. He works on all his projects with his wife Trisha Posner, who is also an author and artist.

Posner was the chief investigative reporter at the Daily Beast. Following the revelation that a number of Posner's stories for the Beast contained portions plagiarized from articles in other publications, Posner resigned from the Beast. According to Posner, the plagiarism was inadvertent and the result of the "compressed deadlines" of the Beast and confusing his assembled research with his own writing in the "master files" he assembled on each story. Allegations of plagiarism also surfaced concerning his latest book, Miami Babylon (October 2009). Posner said the Miami Babylon plagiarism occurred because of a new system of “trailing endnotes”, because an individual he interviewed read one of the plagiarized sources and reiterated it during the interview, and because he mistook other people’s writing for his own after scanning source documents into a computer database. The Miami New Times also found that Posner “seems to add, subtract, or misattribute quotes” and displayed a series of such “apparently altered or misattributed quotes”. For all the examples shown, Posner cited a source article, where an examination of the source showed that the quote given in Posner’s writing was either substantially altered (e.g. words added), never said by the subject, misattributed, or used out of context.

Gerald Posner subsequently hired attorney Mark Lane, threatening litigation against the Miami New Times on grounds of tortious interference (i.e. that its investigation and reporting of this case damaged Posner’s business relationship with his publishers) and emotional distress. In a press release, Posner stated "Although I'm convinced Lee Harvey Oswald assassinated President Kennedy, I've always believed that had Mark Lane represented Oswald, he would have won an acquittal. That's why Mark Lane was the obvious choice as my own attorney." Soon thereafter, the Miami New Times published evidence of additional plagiarism from multiple sources in both Secrets of the Kingdom and Why America Slept. According to Poynter Institute senior scholar Roy Peter Clark "This constitutes plagiarism by any definition I can think of....The capturing of someone else's material that is this extensive cannot, in my opinion, have been done accidentally." Evidence was also presented indicating that Posner had repeatedly “scrubbed” elements of the journalism scandal from his Wikipedia page. According to Posner, the media reports detailing his journalistic transgressions were actually the result of a "coordinated effort" to "discredit my book Miami Babylon" because of the book's "unvarnished and investigative history".