Pickens' spokesman Jay Rosser confirmed the death to The Associated Press. Pickens suffered a series of strokes in 2017 and was hospitalized that July after what he called a "Texas-sized fall."
Governor Greg Abbott issued the following statement: "Although born in Oklahoma, Texas has always considered T. Boone Pickens to be one of our own. He was a passionate man who always stood by his principles on his path to success. T. Boone Pickens' commitment to establishing American energy independence will have a lasting impact on the state of Texas, and the United States of America. Throughout his life and career, he generously gave to charitable causes to advance education, medical research, and humanitarian needs. I ask that all Texans join Cecilia and me in remembering the legacy of T. Boone Pickens, and keep him and his family in their prayers."
An only child who grew up in a small railroad town in Oklahoma, Pickens followed his father into the oil and gas business. After just three years, he formed his own company and built a reputation as a maverick, unafraid to compete against oil-industry giants.
In the 1980s, Pickens switched from drilling for oil to plumbing for riches on Wall Street. He led bids to take over big oil companies including Gulf, Phillips and Unocal, castigating their executives as looking out only for themselves while ignoring the shareholders.
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Even when Pickens and other so-called corporate raiders failed to gain control of their targets, they scored huge payoffs by selling their shares back to the company and dropping their hostile takeover bids.
Later in his career, Pickens championed renewable energy including wind power. He argued that the United States needed to reduce its dependence on foreign oil. He sought out politicians to support his "Pickens Plan," which envisioned an armada of wind turbines across the middle of the country that could generate enough power to free up natural gas for use in vehicles.
"I've been an oilman all my life, but this is one emergency we can't drill our way out of," he said in 2009.
Pickens' advocacy for renewable energy led to some unusual alliances. He had donated to many Republican candidates since the 1980s, and in the 2004 presidential campaign he helped bankroll television ads by a group called Swift Boat Veterans for Truth that attacked Democratic nominee John Kerry. A few years later, Pickens endorsed a Kerry proposal to limit climate change.
Pickens couldn't duplicate his oil riches in renewable energy. In 2009, he scrapped plans for a huge Texas wind farm after running into difficulty getting transmission lines approved, and eventually his renewables business failed.
"It doesn't mean that wind is dead," Pickens said at the time. "It just means we got a little bit too quick off the blocks."
Pickens flirted with marketing water from West Texas, acquiring water rights in the early 2000s in hopes of selling it to thirsty cities. But he couldn't find a buyer, and in 2011 he signed a deal with nearby regional water supplier to sell the water rights beneath 211,000 acres for $103 million.
In 2007, Forbes magazine estimated Pickens' net worth at $3 billion. He eventually slid below $1 billion and off the magazine's list of wealthiest Americans. In 2016, the magazine put his worth at $500 million.
Besides his peripatetic business and political interests, Pickens made huge donations to his alma mater, Oklahoma State University - the football stadium bears his name, and he gave $100 million for endowed faculty positions.
Pickens' foundation gave $50 million each to the University of Texas' M.D. Anderson Cancer Center in Houston and UT Southwestern Medical Center at Dallas. He was among those who signed a "giving pledge" started by billionaire investor Warren Buffet and Microsoft co-founder Bill Gates, promising to donate a majority of his wealth to charity.
"I firmly believe one of the reasons I was put on this Earth was to make money and be generous with it," he said on his website.
Pickens was born in 1928 in Holdenville, Oklahoma. His father was a landman, someone who secures mineral-rights leases for oil and gas drilling. His mother ran a government office that handled gasoline-rationing coupons for a three-county area during World War II.
A child of the Depression, Pickens credited his father with teaching him to take risks and praised his grandmother for lessons in being frugal. If young Boone continued to leave the lights on after leaving a room, she declared, she would hand the electric bill to the boy so he could pay it.
Pickens went to work by age 12, getting a newspaper route. He expanded it by buying the routes on either side of his - marking his first venture into acquisitions.
Although only 5-foot-8, Pickens was a star guard on his high school basketball team in Amarillo, Texas, and earned a sports scholarship to Texas A&M University. He lost the scholarship when he broke an elbow, and he transferred to Oklahoma A&M, now Oklahoma State.
After graduating with a degree in geology, he joined Phillips Petroleum Co., where his father, T. Boone Pickens Sr., was working. The younger Pickens was unhappy with his job from the start.
After just three years, he borrowed some money and found two investors to start his own business, called Petroleum Exploration. That was a predecessor to Mesa Petroleum, an oil and gas company in Amarillo, which Pickens took public in 1964.
By the 1980s, the stock of the major petroleum producers was so cheap that it became cheaper to get new oil reserves by taking over a company than by drilling. Pickens set his sights on acquiring other companies.
In 1984, Mesa Petroleum made a profit of more than $500 million from a hostile bid for Gulf Corp., then the fifth-largest oil company in the United States, when Gulf maneuvered to sell itself instead to Chevron. Before that, Pickens earned $31.5 million by driving Cities Service into the arms of Occidental Petroleum.
Later that year, Pickens launched a bid for his old employer, Phillips Petroleum. It was an unpopular move in Bartlesville, Oklahoma, where Phillips was headquartered. Residents held 24-hour prayer vigils to support the company.
Pickens' methods angered his targets.
"He's only after the almighty buck," G.C. Richardson, a retired executive of Cities Services, said in 1985. "He's nothing but a pirate."
Pickens insisted that he was a friend of ordinary shareholders, who benefited when his forays caused the stock price of a company to rise.
Pickens' star faded in the 1990s. He lost control of debt-ridden Mesa, and his bullishness on natural gas prices turned out to be a costly mistake.
After leaving Mesa, Pickens in 1996 started BP Capital Management, a billion-dollar hedge fund focused on energy commodities and equities that delivered mammoth gains.
There were difficult times in his personal life. In 2005, Pickens looked on as one of his sons, Michael, was arrested on securities-fraud charges - he pleaded guilty and was sentenced to five years' probation and ordered to repay $1.2 million.
Pickens owned a ranch in Texas Panhandle, another in Oklahoma, and a vacation retreat in Palm Springs, California.
After his fall in July 2017, he wrote on Linkedin that he was still mentally strong, but "I clearly am in the fourth quarter."