The cause of death was not disclosed, but Koch Industries said Koch, who lived in New York City, had contended for years with various illnesses, including prostate cancer.
A chemical engineer by training, Koch was an executive in the family-run conglomerate, the Libertarian Party's vice-presidential candidate in 1980 and a major benefactor of educational, medical and cultural organizations.
But he and his brother became best known for building a political network dubbed the "Kochtopus" for its many-tentacled support of conservative and libertarian causes, candidates and think tanks, including the Cato Institute.
The brothers in 2004 founded the anti-tax, small-government group Americans for Prosperity, which remains one of the most powerful conservative organizations in U.S. politics, and they were an important influence on the tea party movement.
While celebrated on the right, the Koch brothers have been vilified by Democrats and others who see them as a dark and conspiratorial force, the embodiment of fat-cat capitalism and the corrupting influence of corporate money in American politics.
"I was taught from a young age that involvement in the public discourse is a civic duty," David Koch wrote in a 2012 op-ed in the New York Post. "Each of us has a right - indeed, a responsibility, at times - to make his or her views known to the larger community in order to better form it as a whole. While we may not always get what we want, the exchange of ideas betters the nation in the process."
Some prominent Republicans praised his legacy upon his death, with libertarian-leaning Sen. Rand Paul of Kentucky saying "his many contributions will have lasting impact on our country."
On the other side of the political divide over Koch, Beth Rotman of the government watchdog group Common Cause said the Kochs and their network of like-minded wealthy donors "undermined so many important American values over the past several decades as part of the Kochs' attempted corporate takeover of American politics."
Among other things, the Kochs and their company bankrolled a decades-long movement to cast doubt on man-made climate change and to thwart efforts to fight global warming through reducing greenhouse gas emissions.
"David Koch will likely be remembered as one of a small handful of individuals who singlehandedly thwarted efforts to act on climate change and other pressing environmental threats aimed at preserving our planet for future generations," said Pennsylvania State University climate scientist Michael Mann.
The brothers also invested heavily in fighting President Barack Obama's health care overhaul. They pressed to bring conservative voices to college campuses. And they developed a nationwide grassroots network backing conservative causes and candidates at the state and national levels.
They drew the line at Donald Trump. The Kochs refused to endorse him in 2016, warning that his protectionist trade policies, among other priorities, weren't sufficiently conservative.
Trump, for his part, tweeted last year that the brothers were a "total joke in real Republican circles," calling them "two nice guys with bad ideas."
David Koch had stepped away from a leadership role in recent years because of declining health, including a decades-long battle with prostate cancer, and his brother became the network's public face.
In an interview after the 2012 Republican convention, his mind was on his legacy.
"When I pass on," he told The Weekly Standard, "I want people to say he did a lot of good things, he made a real difference, he saved a lot of lives in cancer research."
Koch donated $100 million in 2007 to create a cancer research institute at the Massachusetts Institute of Technology. He also gave millions to Johns Hopkins University in Baltimore, the Memorial Sloan-Kettering Cancer Center in New York, the M.D. Anderson Cancer in Houston and other institutions.
The Lincoln Center theater that houses the New York City Ballet became the David H. Koch Theater in 2008 after he gave $100 million. Parts of the Smithsonian's National Museum of Natural History are named for him after he contributed a total of $50 million for exhibits devoted to dinosaur fossils and human evolution.
He said his philanthropy was fueled by a brush with death during a 1991 collision of two airliners at the Los Angeles airport. Thirty-four people were killed. Koch spent two days in intensive care with smoke inhalation.
"I felt that the good Lord was sitting on my shoulder and that he helped save my life because he wanted me to do good works and become a good citizen," he told Barbara Walters in 2014.
Charles and David Koch, each with an estimated net worth of $50.5 billion, were tied for 11th place in 2019 on the Forbes 500 list of the nation's richest men.
Their father, Fred, in 1940 co-founded the company that later became Koch Industries. The Wichita, Kansas-based conglomerate has vast holdings in oil refineries, paper mills, fertilizer plants, cattle ranches and other ventures. It is the company behind Stainmaster carpeting, Brawny paper towels and Dixie cups.
It has drawn fire for years from environmental advocates and researchers. Koch Industries in 2000 paid $35 million - then the largest civil fine ever levied under the federal Clean Water Act - to settle lawsuits over oil pipeline leaks into lakes and streams in six states.
The University of Massachusetts Amherst's Political Economy Research Institute ranks Koch Industries one of the top 25 polluters in the U.S.
David Koch, who held degrees from MIT, served on Koch Industries' board and was also CEO of a Koch chemical subsidiary. He retired from the company as executive vice president in 2018.
Two other Koch brothers, Frederick and Bill Koch, came out on the losing end of a power struggle for control of the company's board. They sold their stake in Koch Industries in 1983, later unsuccessfully claiming in a lawsuit that they were cheated out of more than $1 billion. David and Bill Koch were twins.
David Koch is survived by his wife, Julia Flesher, and their three children.
On Friday, Charles Koch said of his younger brother: "The significance of David's generosity is best captured in the words of Adam Smith, who wrote, 'to indulge our benevolent affections, constitutes the perfection of human nature.'"
Associated Press writers Seth Borenstein in Washington and Patrick Whittle in Portland, Maine, contributed.
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