Gavin MacFadyen, right, an investigative journalist and documentarian, with the WikiLeaks founder Julian Assange in 2011. Credit John Stillwell/Press Association
Gavin MacFadyen, an American investigative journalist who became an early mentor and defender of the founder of WikiLeaks, Julian Assange, died on Saturday in London, where he lived and spent much of his professional life. He was 76.
The cause was lung cancer, his wife, Susan Benn, said.
Since the 1970s, Mr. MacFadyen produced and directed scores of television documentaries on a wide range of subjects, including neo-Nazi violence, child labor, nuclear proliferation and industrial accidents. Sometimes he worked in disguise.
He also co-founded the nonprofit Center for Investigative Journalism in London in 2003, a training program in skeptical reporting, and WhistleblowersUK, a support group for tipsters. He was a director of WikiLeaks and, with his wife and another journalist, John Pilger, formed the Julian Assange Legal Defense Committee.
Mr. Assange, an Australian computer programmer, founded WikiLeaks in 2006 and published millions of secret documents, many supplied by Chelsea Manning, a United States Army intelligence analyst. Mr. Assange has been under investigation by the American government and is wanted for questioning about rape allegations in Sweden. He has found refuge in the Ecuadorean Embassy in London to avoid extradition.
Immediately after Mr. MacFadyen’s death, WikiLeaks issued this Twitter post: “Gavin Macfadyen, beloved director of WikiLeaks, now takes his fists and his fight to battle God. Sock it to him, forever, Gavin.” It was signed “JA.”
In his book “WikiLeaks: News in the Networked Era” (2012), Charlie Beckett wrote that Mr. MacFadyen “was a core WikiLeaks supporter who had offered the services of interns, facilities and even on occasion his sofa to the team.” Mr. Assange moved into Mr. MacFadyen’s London townhouse in 2010, bringing with him only three pairs of socks.
Elaine Potter, a philanthropist and co-founder of the Bureau of Investigative Journalism, based in London, said Mr. MacFadyen had been driven by “passion, politics and curiosity.”
“He recognized the significance of WikiLeaks and made contact from the moment they arrived on the internet,” she said. “He became obsessed with providing support for whistle-blowers.”
Mr. Assange and WikiLeaks have been thrust into the spotlight in recent months by the release of thousands of Democratic National Committee emails amid suspicions by United States officials that the files were hacked by the Russian government, possibly to influence the American elections.
But Mr. MacFadyen maintained that Western news organizations had uncritically published material provided by the Central Intelligence Agency and that the fundamental question was whether the information was true and in the public interest, rather than its source.
“His commitment to exposing the true nature of power was his life force,” Ms. Benn, his wife, said. “He spearheaded the creation of a journalistic landscape which has irrevocably lifted the bar for ethical and hard-hitting reporting.”
She added: “Gavin worked tirelessly to hold power to account. He once said, ‘Good journalism is always political journalism.’”
Mr. MacFadyen was born Gavin Hall Galter on Jan. 1, 1940, in Greeley, Colo., and grew up in Chicago. He never knew his father, and he adopted the surname of his stepfather, a medical researcher. His mother was a pianist.
He worked as a union organizer and demonstrated for civil rights before moving to Britain, where he graduated from the London School of Film Technique (now the London Film School).
Afterward, he created a documentary film group to chronicle the political turmoil in the United States during the late 1960s for the BBC, covering anti-Vietnam War protests, race riots and the police clash with demonstrators at the Democratic National Convention in Chicago in 1968.
He went on to cover the war in Nicaragua between the right-wing contra rebels and the Marxist Sandinista government in the 1980s.
Collaborating with the director Michael Mann, he played Boreksco, a crooked police officer, in Mr. Mann’s 1981 debut feature film, “Thief,” and was a technical adviser to “The Insider,” Mr. Mann’s 1999 film about Jeffrey Wigand, a tobacco company whistle-blower, starring Russell Crowe and Al Pacino.
He produced documentaries for the BBC, Granada Television and ABC-TV and for Frontline on PBS. He was a visiting professor at City, University of London.
In addition to Ms. Benn, he is survived by a son, Michael, from his first marriage, to Virginia Daum, which ended in divorce; three stepdaughters, Sarah Saunders, Deborah Ramsay and Samantha McLean; and six grandchildren.
In an interview with Delft University of Technology in the Netherlands in 2009, Mr. MacFadyen drew a distinction between being a witness and bearing witness, and defined a whistle-blower or an informer as someone who values the truth because it will affect the future.
“The wealthy and powerful often are unhappy about telling the truth,” he said. “There is a famous story that if you give a poor man on the street a dollar, you’re considered a good Christian, but if you ask, ‘Why is that man poor?’ you’ll go to prison.”
Journalism, he suggested, was essentially pursuing answers to basic questions: “A building has blown up. Great. We need to know that — very important. Who did it? Why did it blow up? What are the consequences? Who was killed? Who benefits from this? Those are the questions I want to ask.”
He added: “The more we know, the more we can control an event or stop those events. To understand the events, not to cry, not to laugh, but to understand.”
A version of this article appears in print on October 28, 2016, on page B14 of the New York edition with the headline: Gavin MacFadyen, Mentor and Defender Of WikiLeaks’s Founder, Is Dead at 76.