Pickens died of natural causes at his home in Texas, surrounded by family and friends.
Pickens is survived by five children and 11 grandchildren.
President George W. Bush on Wednesday sent his condolences to Pickens’ family, calling the businessman “bold, imaginative and daring.”
A health and fitness advocate who famously challenged President Barack Obama to a 1-hour workout, he suffered a series of strokes and 2017, but was able to battle back.
Pickens also influenced the Trump administration, having had numerous discussions regarding energy policy with Donald Trump during his presidential campaign.
Pickens was a self-made oil mogul, who had a lasting influence on Wall Street, too. He was one of the first businessmen to push for shareholder rights, advocating for executives to be compensated with stock, as reported by The New York Times.
Pickens worked for Phillips Petroleum early in his career. He founded Mesa Petroleum in the 1950s.
He was known as a corporate oil raider -- he regularly bought a large amount of shares in companies that appeared to be undervalued.
He was a philanthropist who donated more than $1 billion to charity, as well as a major Republican Party donor.
The oil tycoon was a big Oklahoma State University fan – his alma mater. The football stadium is named after him.