Ellsberg was a military analyst when he released thousands of documents to US media in 1971 that revealed successive United States administrations had lied to the public about the Vietnam war.

The 7,000 classified pages determined that, contrary to the public assertions of US government officials, the conflict was unwinnable.

The leak was recounted in the 2017 Hollywood thriller "The Post," which detailed the nail-biting behind-the-scenes story of the papers' publication.

Ellsberg announced in March that doctors had told him on February 17 that he had terminal pancreatic cancer and only around six months to live.

"He was not in pain, and was surrounded by loving family," his wife and children said in a statement announcing his death.

They highlighted that his last months had been well spent despite his illness.

"He was thrilled to be able to give up the salt-free diet his doctor had him on for five years," they said.

"Hot chocolate, croissants, cake, poppyseed bagels, and lox gave him extra pleasure in these final months.

"He also enjoyed re-watching his favorite movies, including several viewings of his all-time favorite, 'Butch Cassidy and the Sundance Kid," added his wife Patricia, sons Robert and Michael, and daughter Mary.

The New York Times initially published excerpts of the Pentagon Papers until the administration of President Richard Nixon obtained a court injunction barring the newspaper from continuing to do so on national security grounds. The Washington Post then took up the mantle.

Ellsberg was charged under America's Espionage Act but the case ended in a mistrial in 1973 after illegal evidence gathering by the government came to light.

- 'Patriotic truth-teller' -

Announcing his diagnosis on March 3, Ellsberg reflected on his history-changing actions.

"When I copied the Pentagon Papers in 1969, I had every reason to think I would be spending the rest of my life behind bars," he wrote.

"It was a fate I would gladly have accepted if it meant hastening the end of the Vietnam War, unlikely as that seemed (and was).

"Yet in the end that action -- in ways I could not have foreseen, due to Nixon's illegal responses -- did have an impact on shortening the war," Ellsberg added.

He continued to speak out against war through the remainder of his life, repeatedly criticizing US interventions overseas, particularly in Afghanistan and Iraq.

"What we've done to the Middle East has been hell," he told the publication Democracy Now in 2018.

Ellsberg was a staunch anti-nuclear weapons campaigner. In 2017, he published a massive tome about the nuclear threat seen from the inside, titled "The Doomsday Machine: Confessions of a Nuclear War Planner."

Ellsberg continued his activism even after his diagnosis, speaking to reporters about the continuing threat of nuclear war, particularly the threat posed by Russia's invasion of Ukraine.

"Daniel was a seeker of truth and a patriotic truth-teller, an antiwar activist, a beloved husband, father, grandfather, and great-grandfather, a dear friend to many, and an inspiration to countless more. He will be dearly missed by all of us," his family said.