In the rain forest and savanna of British Guiana, an accident could mean death. When Stan Brock was thrown against the side of a corral, severely injured while training a wild horse, the British-born teenager was told it would take 26 days to reach the closest doctor. It would have been easier, he later observed, for a wounded Apollo astronaut to make the three-day trip home from the moon.

Mr. Brock remained in the wilderness, convalescing among the Wapishana Indians who taught him how to herd cattle and ride barefoot. But he also began to nurture an abiding interest in health care for secluded and underserved populations, and three decades later — in 1985 — established Remote Area Medical.

The nonprofit was intended to hold free clinics in the developing world, but soon after it started, Mr. Brock had a revelation: “There were people like the Wapishanas, who were 26 days on foot from the nearest doctor, in a place like Chicago, where there was a doctor just around the corner but they simply couldn’t afford to go there.”

Mr. Brock was 82 when he died Aug. 29 at RAM’s headquarters in Rockford, Tenn., where he slept on a grass mat next to his desk and oversaw an organization that has treated more than 740,000 people and delivered an estimated $120 million of free medical services, primarily in the United States.

At one typical clinic last year, at a fairground in Wise, Va., 1,400 volunteers treated 2,300 patients, some of whom camped out for three days to make sure they received treatment. One man needed 18 teeth pulled. Another needed help applying for a kidney transplant. Nearly all of them were part of the estimated 12.2 percent of Americans without health insurance.

“The health fair reminded me of scenes I’ve witnessed in refu­gee camps in South Sudan,” wrote New York Times columnist Nicholas Kristof. Its success, one volunteer physician told him, was “an indictment of our health care system.”

Notable deaths in 2018 and 2019: Nipsey Hussle, George H.W. Bush, Stan Lee, John McCain, Aretha Franklin and other famous faces we’ve lost

Nipsey Hussle, a Grammy nominated rapper who sought to revive South Los Angeles, died at age 33 on March 31. Read the obituary (Matt Winkelmeyer/Getty Images for Warner Music)

Mr. Brock became something of a health-care crusader in recent years, calling for President Trump to visit a clinic and see the “appalling” health of Americans from rural Appalachia to Puerto Rico, and in cities such as Los Angeles and Seattle. He also maintained RAM’s work abroad, with programs that include clinic “expeditions” in Haiti and the Philippines, relief efforts for hurricane and earthquake victims, and an airplane-ambulance program in Guyana.

His work was all the more remarkable given his background — not as a physician, but as the manager of a sprawling cattle ranch in the Amazon and as an ad­ven­ture star in movies and television, with credentials that included a black belt in taekwondo.

A former co-host of “Mutual of Omaha’s Wild Kingdom,” the popular nature series starring Marlin Perkins, he was shown lassoing buffalo and wrestling anacondas and was credited with discovering a new bat species that now bears his name.

Even after quitting television and selling his belongings to found RAM, he sported an open-neck khaki shirt and bomber jacket, relics from his days as a globe-trotting adventurer. He professed to hating hot water and loathed sitting down. Dubbed a “medical monk,” he reportedly subsisted on a diet of rice, beans, bananas, oatmeal and the occasional can of tuna.

In an interview for “Medicine Man,” a forthcoming documentary, former RAM executive director Karen Wilson said Mr. Brock’s ascetic lifestyle was driven entirely by his work for the organization. “There are no hobbies, there are no vacations, there is nothing in his life except Remote Area Medical.”

Stanley Edmunde Brock was born in Preston, in the northern English county of Lancashire, on April 21, 1936. His father worked for the British government, and the family lived in 26 different places, by his mother’s count, before receiving an assignment in what is now Guyana.

Mr. Brock remained in England, where he said he was bullied at boarding school and dreamed of distant exploits in the jungle. At 16, he boarded a ship to the colonial capital of Georgetown for what was supposed to be a short trip to see his parents. He never returned to school and by 1953 was at the Dadanawa Ranch in the country’s interior, where he developed a talent for horse riding, worked as a vaquero and “learned to live as man was originally intended to live on this planet.”

He kept wild animals at home, wrestling with a friendly puma cub named Leemo that was eventually killed by another pet, an ocelot he named Beano, and because of his skills with a lasso attracted the attention of “Wild Kingdom” creator Don Meier.

Mr. Brock joined the show as co-host in 1967 and, moving to the United States, stayed with the program for five years.

He briefly starred in his own nature-adventure series, “Stan Brock’s Expedition Danger,” and was featured in a pair of ad­ven­ture films by “Flipper” producer Ivan Tors — including “Galyon” (1980), in which he played a “one-man war machine” who rescues a family from South American terrorists.

Mr. Brock was working as a designer and consultant for zoos when he landed in Knoxville, Tenn., where he and RAM were long based before moving to Rockford. The organization’s first clinic opened in Mexico, and it began shifting its efforts to the United States in 1992, when a clinic opened in Sneedville, Tenn.

Mr. Brock sometimes flew a World War II-era cargo plane to the clinics, ferrying supplies that included donated dental chairs, eye charts and paper gowns. He was also credited with pushing for a Tennessee state law that enables out-of-state practitioners to volunteer with clinics in Tennessee. Similar laws now exist in about a dozen states, said RAM spokesman Robert Lambert.

Mr. Brock had recently suffered a stroke, Lambert said, and is survived by a brother.

He was previously married, he told Britain’s Independent newspaper in 2014, but found himself too busy with his organization to start a family.

“Would I like to have children . . .? Yes. But I’ve got thousands of them now. We did house calls on two of them just last week. A disabled 15-month-old living out in a shack in the woods in Tennessee, gonna require lifelong care, with a single mother who was brutally beaten by her husband.”

“For the I-don’t-know-how-many years left I have on this earth, this isn’t the time to be sitting on the porch looking up at the Smoky Mountains,” he added. “I might as well do something.”