George Butler, an adventurous filmmaker who deftly explored the subculture of bodybuilding in “Pumping Iron,” a documentary with the then little-known Arnold Schwarzenegger as its charismatic center, died on Oct. 21 at his home in Holderness, N.H. He was 78.

The cause was pneumonia, his son Desmond said.

“Pumping Iron” (1977) started the British-born Mr. Butler on an eclectic journey as a documentarian: He went on to direct films about Ernest Shackleton’s expedition to Antarctica; endangered Bengal tigers in the Sundarbans, a tidal mangrove forest in India and Bangladesh; the exploration of Mars by robotic vehicle; and his longtime friend John Kerry’s presidential campaign of 2004.

“He said movies should take people to places they couldn’t imagine, not just places where they hadn’t been,” Caroline Alexander, his longtime companion and business partner and the writer of five of his documentaries, said by phone.

Mr. Butler’s best-known film is the popular documentary (with some scripted sequences) “Pumping Iron,” which he directed with Robert Fiore. It is credited with helping bodybuilders escape their niche as physical curiosities and win recognition as serious athletes.

Mr. Butler told The Daily News of New York in 1977 that there was a “myth” that bodybuilders were “uncoordinated, dumb, narcissistic muscleheads,” but that they were actually “adept at other sports — some at a professional level.”

“Pumping Iron” focused on a group of bodybuilders as they worked out at Gold’s Gym in Venice Beach, Calif., and competed in 1975, some for the title of Mr. Universe and some for Mr. Olympia, in Pretoria, South Africa. The film paid particular attention to the intense rivalry between Mr. Schwarzenegger, a five-time Mr. Olympia, and the shy Lou Ferrigno, who was cast soon afterward in the title role of the television series “The Incredible Hulk.”

Kevin Thomas of The Los Angeles Times praised “Pumping Iron” for treating the bodybuilders “with neither compassion nor ridicule but rather a steadfast, cool detachment — even when they themselves are being nakedly manipulative — which makes for a slick, shrewdly calculated, highly amusing and enjoyable experience.”

George Tyssen Butler was born on Oct. 12, 1942, in Chester, England, and grew up in Wales, Somalia, Kenya and Jamaica. His father, Desmond, was an Irish-born British Army officer who later ran a plantation and an Avis rental car franchise in Jamaica. His mother, Dorothy (West) Butler, owned a catering business and rental properties in Jamaica.

George’s sense of adventure was stoked in Somalia, where he drank camel’s milk and hunted for dinner with his father. While living in Jamaica, he lifted weights at a gym in Montego Bay.

He graduated from the Groton School in Massachusetts, then earned a bachelor’s degree in English at the University of North Carolina and a master’s in creative writing from Hollins College (now University) in Roanoke, Va. He subsequently joined Vista (now AmeriCorps Vista), the national service program, in Detroit, where he started a community newspaper and began taking photographs of the city after devastating riots.

His friendship with Mr. Kerry, a future Massachusetts senator and secretary of state, began in 1964 when they met at a party. In 1971, he accompanied Mr. Kerry when he gave emotional testimony against the Vietnam War to the Senate Foreign Relations Committee, and he was an editor and photographer for “The New Soldier,” Mr. Kerry’s book about protests in Washington by the organization Vietnam Veterans Against the War.

Mr. Butler’s involvement with bodybuilding began in the early 1970s when he took pictures of competitions for Life and The Village Voice. He and Charles Gaines, the author of “Stay Hungry,” a 1972 novel about bodybuilding, teamed up for a Sports Illustrated article about a competition in Holyoke, Mass. At another event, at the Brooklyn Academy of Music, Mr. Butler watched Mr. Schwarzenegger pose for a rapt audience.

“When he came out for that, the crowd just erupted with clapping and cheering like I’d never seen before,” Mr. Butler told Muscular Development magazine in 2016.

In a phone interview, Mr. Gaines said: “George and I were similarly impressed with Arnold, who we thought would be something more than just a bodybuilding champion. We decided the subject was interesting enough to warrant a book.”

Their book, “Pumping Iron: The Art and Sport of Bodybuilding” (1974) was a success — by 1982, it had sold 258,000 copies — and led to the making of the documentary, for which Mr. Butler and Mr. Gaines wrote the script.

But when potential investors were shown the 10-minute test film that Mr. Butler had shot, featuring Mr. Schwarzenegger, the response was silence. He told Muscular Development that the playwright Romulus Linney stood up to say, “If you make a movie about this Arnold person, we will laugh you off 42nd Street.”

Mr. Butler raised several thousand dollars from attendees at a bodybuilding exhibition he put on at the Whitney Museum in Manhattan, featuring Mr. Schwarzenegger and others, and raised the rest from a variety of other sources, including his mother-in-law.

Mr. Schwarzenegger, in a statement after Mr. Butler’s death, praised him for his “fantastic eye” and said he had been “a force for the sport of bodybuilding and the fitness crusade.”

Mr. Butler’s other films included “In the Blood” (1989), which connected a big-game hunt by Teddy Roosevelt in 1909 with a modern expedition in Africa by Mr. Butler and his son Tyssen, and “The Endurance: Shackleton’s Legendary Antarctic Adventure” (2000). Among his other projects was a book of photographs of Mr. Schwarzenegger

He adapted the Shackleton movie for the Imax screen as “Shackleton’s Antarctic Adventure” (2001), adding an extra element by enlisting three mountaineers to retrace the land route of the expedition.

In 2004 he produced and directed “Going Upriver,” about Mr. Kerry’s Navy service and antiwar activism, for which Mr. Butler drew on some of the 6,000 photographs he had taken of Mr. Kerry.

He went on to make a second Imax film, “Roving Mars” (2006), about the journey of two rovers sent by NASA to explore the planet and the images they transmitted back to Earth; “The Lord God Bird” (2008), about the futile search for the rare ivory-billed woodpecker, which was declared extinct last month; and “Tiger, Tiger,” which followed the big-cat conservationist Alan Rabinowitz, who had been diagnosed with leukemia, to the Sundarbans. Mr. Butler himself was struggling with symptoms of Parkinson’s disease while making “Tiger, Tiger.”

An Imax version of “Tiger, Tiger” is expected to be released next year.

“Alan was on what might have been his last quest,” Ms. Alexander, his partner, said, “doing what he could for this animal, which was under a death sentence.” Mr. Rabinowitz died in 2018.

In addition to Ms. Alexander and his two sons, Mr. Butler is survived by a brother, Richard, and six grandchildren. His marriage to Victoria Leiter ended in divorce.

Mr. Butler was not finished with bodybuilding after “Pumping Iron.” In 1985, he focused on women bodybuilders like Rachel McLish and Bev Francis in “Pumping Iron II: The Women.”

In her book “Moving Beyond Words” (1994), Gloria Steinem called that movie a “historic mindblower” and said that it “began a revolution in our ideas about women’s bodies that is still going on.”