Ronnie Cummins, a ponytailed activist who became one of the country’s leading advocates for organic food and a leading critic of genetically modified food, died on April 26 in San Miguel de Allende, Mexico, where he lived and worked part-time. He was 76.

Rose Welch, his wife and partner in starting the Organic Consumers Association, an advocacy and informational organization, said his death, which was not widely reported at the time, was caused by bone and lymph cancer.

Mr. Cummins was a lifelong activist and protester, beginning with his opposing the Vietnam War and nuclear power. He settled on organic food activism in the 1990s after he was hired as a director of the Pure Food Campaign, a lobbying group that sought to broaden awareness of the dangers of genetically engineered food while pushing for responsible labeling and government testing.

Mr. Cummins worked in the field for the campaign, raising alarm at rallies and supermarkets about the perils of foods using genetically modified ingredients. He handed out leaflets, wrote opinion articles and answered consumers’ questions as a campaign spokesman.

He also worked for the Beyond Beef campaign, aimed at reducing beef consumption and promoting safer methods of cattle production. Both campaigns were founded by the environmental activist and social theorist Jeremy Rifkin.

Mr. Cummins “was a tough guy who could be an activist and also step back and do the intellectual homework behind what we were doing,” Mr. Rifkin said in a phone interview.

“Too often activists burn out after starting out with high expectations,” he added. “But Ronnie could write, research, reflect and be open to all points of view.”

One of Mr. Cummins’s frequent targets was recombinant bovine somatotropin, or bovine growth hormone, a genetically engineered hormone, produced by Monsanto, that stimulates milk production in cows.

On the first day that farmers were allowed to sell milk from cows injected with the hormone, in 1994, Mr. Cummins told The Associated Press that “if we don’t slow down the technology of change with genetically engineered additives, we will be making a very major mistake in terms of human health, animal health and the survival of family farms.”

He continued to rail about milk produced by hormone-treated cows after he and Ms. Welch started the Organic Consumers Association, based in Finland, Minn., in 1998.

“Recombinant bovine growth hormone is bad for dairy cows, literally burning them out in three or four years, causing terrible physical stress and a long list of medical problems including reproductive complications,” Mr. Cummins wrote in The Fresno Bee in 2008.

He relished battling with major brands. In 2001, he raised doubt about Starbucks’s promise not to use milk products with the hormone by asking to see its promise in writing. (The company eventually complied in 2007.) He warned about a “sneak attack engineered by the likes of Kraft, Dean Foods and Smucker’s.” To pressure companies using modified beet sugar, he threatened a protest against Hershey.

Though there are unresolved questions about the effect of genetically modified organisms on biodiversity, there is a near-universal consensus among scientists that genetically modified foods are safe to eat.

Most consumers do not share that view, however, a skepticism due in large part to the efforts of activists like Mr. Cummins.

The safety of genetically modified food “is like global climate change, where 99 percent of scientists believe in it,” Pamela Ronald, a plant pathology professor at the University of California, Davis, told The Roanoke Times in 2013.

She added, “You have scientists around the world who say genetically engineered crops are safe to eat — and then you have Ronnie Cummins.”

Mr. Cummins was born Adrian Alton Abel on Oct. 28, 1946, in Jefferson, Tex., about 20 miles from the Louisiana border. His father, Jack, was an accountant for Gulf Oil in Port Arthur, Texas, in the heart of the state’s oil industry. His mother, Elise (Stout) Abel, was a homemaker who died by suicide in 1951.

In his 20s, Adrian changed his name to Ronnie Cummins, the name of a boy who was also born in 1946 and who died in 1954. Ms. Welch said he changed his name because he feared reprisals from the Ku Klux Klan for his antiwar activities at Rice University in Houston, where he had majored in English and philosophy and graduated with a bachelor’s degree in 1969.

Ms. Welch said she did not know why her husband took the Cummins boy’s name in particular. She said he told her that he did not have a criminal record that he was seeking to hide with a new identity. His brother, Jack Abel Jr., said by phone that the story behind the name change “is so personal I can’t share it.”

In addition to his wife and brother, Mr. Cummins is survived by his son, Adrian Cummins Welch; and his sisters, Molly Travis and Bonnie Abel.

Adrian grew up among refineries and later recalled catching fish polluted by oil. But he also spent idyllic summers on his maternal grandparents’ farm, where he took care of animals and gathered eggs.

“My life experience has taught me that money rules and power corrupts, and that putting profits before people and environmental health is not only wrong but deadly,” he wrote in his book “Grassroots Rising: A Call to Action on Climate, Farming, Food and Green New Deal” (2020). “Organized grass-roots power can make a big difference,” he added, “whether we’re talking about public consciousness, marketplace pressure or politics and public policy.”

As a career, activism didn’t pay the bills, so he earned a living over the years as a newsstand owner at the University of Minnesota, the director of a food co-op in Burnsdale, Minn., outside Minneapolis, and a house painter. Ms. Welch waited tables.

“He was pretty much a hippie,” she said in a phone interview.

Both went to work for Mr. Rifkin in the 1990s, Mr. Cummins as a director, Ms. Welch as a campaign manager. They left to start the Organic Consumers Association, which supports enforcement of the U.S. Department of Agriculture’s organic food standards, produces educational material for organic consumers and businesses, and encourages public pressure campaigns on organic food issues.

The “hippie” was finally earning a real salary — $112,900 in 2021.

The O.C.A. has spun off two organizations: the Mexico-based Via Orgánica, an agroecology farm school and research center, in 2009, and, in 2014, Regeneration International, which advances ways to develop farming practices that rebuild degraded soil.

In the view of André Leu, the international director of Regeneration International, Mr. Cummins had stood up to “the powerful elite who were monopolizing power and wealth” and were “undermining democracy, fair wages, healthy food, peace, the climate, and the environment.”

A longtime goal of Mr. Cummins’s was for the government to require labeling on genetically modified food. He fought for ballot initiatives in several states and won his first major victory in Vermont, in 2014, when it became the first state to pass a labeling law.

Faced with the prospect of a patchwork of state laws, Congress passed a sweeping federal labeling law in 2016.

But Mr. Cummins did not consider it a victory.

The law, which superseded the tougher Vermont legislation, gave companies the option of using an icon or a scannable QR code that would direct consumers to a website, instead of having to spell out the information on the package. And some foods, like highly refined sugars and oils, were exempt from the labeling requirement.

Mr. Cummins, in an article on his website, called brands like Organic Valley and Stonyfield Farms “organic traitors” and accused the Grocery Manufacturers Association, the Whole Foods supermarket chain “and a cabal of sellout, nonprofit organizations” of surrendering “to Monsanto and a corporate agribusiness” by backing the legislation.

“In other words business as usual,” he added, then used a buzzword for genetically modified products — “Shut up and eat your Frankenfoods.”

Sheelagh McNeill contributed research.