The Oklahoma-born tycoon, known for his folksy speech and ruthless business acumen, died surrounded by his family and friends, said longtime spokesman Jay Rosser.
"He was coming into the office every day until Monday," said Rosser. "He stayed active running his own portfolio."
Pickens closed his BP Capital energy hedge fund after suffering a stroke in late 2016 and due to the effects of a fall in mid-2017 that required hospitalization.
"Trading oil is not as intriguing to me as it once was,” Pickens wrote more than 18 months ago on his boonepickens.com website to announce the closure of the fund. "It’s no secret the past year has not been good to me, from a health perspective or a financial one."
Pickens was remembered for his contributions to advances in medical research, athletic and educational initiatives, and for advocating for increased domestic use of abundant natural gas.
"He generously shared his success with institutions and communities across Texas and Oklahoma," said former President George W. Bush.
Pickens' commitment to energy projects "will have a lasting impact on the state of Texas, and the United States of America," added Texas Governor Greg Abbott.
In late 2017, Pickens had put his 65,000-acre (26,000-hectare) Mesa Vista Ranch in the Texas Panhandle up for sale at a price of $250 million. In addition to a castle-like main building, the ranch had its own airfield, art gallery, pub, chapel and two-story dog kennel.
Pickens ran energy firms, hedge funds and wind farms, offering forecasts on oil prices and in 2008, when his fortune was estimated at $3 billion by Forbes magazine, announced the Pickens Plan for an energy independent United States. He promoted the plan in television ads and interviews and testified about it before Congress, bemoaning reliance on foreign oil and pushing the potential of wind energy.
Pickens had aspirations of creating the world's biggest wind farm in the Panhandle with the capability of powering 1.3 millions homes at a cost of $10 billion. By 2010, however, he had given up on a big-scale wind farm due to difficulties with financing and a lack of transmission lines.
Pickens had gained notoriety in the 1980s as a consummate dealmaker who launched audacious bids to take over giant oil companies including his former employer Phillips Petroleum Co, as well as Gulf Oil Corp and Unocal Corp with leveraged buyouts.
He failed on all the big takeover bids but made millions of dollars in the process. With Gulf Oil, his bid drove the nation's fifth largest oil company into a record $13.2 billion merger with Chevron Corp and his bold moves landed him on the covers of magazines such as Forbes and Time.
"There is nothing better than being the underdog," he wrote in his 2008 book "The First Billion is the Hardest."
"The more people count me out, the more I count myself in."
Dubbed the "oracle of oil" in media outlets, Pickens portrayed himself as a tireless campaigner for shareholder rights and repeatedly argued that America's managers had become more concerned with the four P's - pay, perks, power and prestige - than with making money for the company's real owners.
Thomas Boone Pickens Jr. was born on May 22, 1928, in Holdenville, Oklahoma, and in the late 1930s moved with his family to Texas.
Graduating with a geology degree from what later would become Oklahoma State University, Pickens worked for Phillips Petroleum for about three years before borrowing $2,500 and going out on his own. He and two investors formed an oil and gas firm called Petroleum Exploration Inc.
The firm would later became Mesa Petroleum, which at one time, was one of the world's largest independent oil companies, with assets of more than $2 billion in 1980, according to the Texas State Historical Association.
In 1996, Pickens, announced his retirement from Mesa. The move came after a long battle with dissident shareholders, who were angry at the company's mounting debts. Critics say he held bitter grudges against those he thought had crossed him.
In the late 1980s, Pickens shook up corporate Japan when he tried to take over Koito Manufacturing Co Ltd, a Tokyo-based automotive parts maker. He became the biggest single shareowner in the firm with a holding of 20.2%, about 1% more than Toyota Motor Corp.
After butting heads with corporate management and investors for some two years over the raid, a rare occurrence in Japan, he sold back his holdings.
Pickens also made waves politically, helping finance accusations by the Swift Boat Veterans for Truth against Democratic Senator John Kerry in his 2004 presidential election campaign, which questioned aspects of the politician's military service in the Vietnam War.
Pickens contributed hundreds of millions of dollars to Oklahoma State University and set up a foundation that bears his name, which made major donations to medical institutions in Texas. In 2005 the school renamed its football stadium for him.
He was known for his nuggets of homespun business wisdom known as Boone-isms, such as, "A fool with a plan can beat a genius with no plan."
Pickens is survived by his five children, 11 grandchildren and several great-grandchildren.
(Reporting By Staff; Editing by Marguerita Choy)