Legendary oil man T. Boone Pickens, whose investments helped shape the American energy industry going back to the 1950s, has died at the age of 91. Pickens, a long-time Dallas resident, died of natural causes on Wednesday, spokesman Jay Rosser said in a statement on Pickens’ website. He had a series of strokes and suffered from head injuries after falling in 2017. Pickens announced his retirement from the oil and gas industry in January 2018 due to his poor health. In a letter posted to LinkedIn at the time he wrote that “it’s no secret the past year has not been good to me” adding that “it’s time to start making new plans and setting new priorities.” A longtime maverick in the US energy industry, Pickens began a campaign in 2008 to help the US lessen its dependence on oil from OPEC nations. In 2007, he had unveiled a proposal for a large wind farm project. The plan failed, mainly due to a steep drop in natural gas prices that made wind and other sources of renewable energy more expensive. “I lost $150 million in the wind business,” he complained at a 2015 SALT investor conference. Pickens rose to prominence in the 1980s as a so-called corporate raider, making several big investments in oil companies with the hopes of forcing executives to make big changes to boost shareholder value. “I’m perhaps best known for my attempted corporate takeovers throughout the 1980s. I have always believed that maintaining the status quo inevitably leads to failure,” Pickens wrote in an op-ed for Forbes in 2017. “Back then, the notion that shareholders own the companies and managements were employees was foreign to big oil companies that would rather operate like empires. I was hell-bent on shaking things up. I was a disrupter before disrupters were cool.” Despite the boom in the shale gas business over the past decade, Pickens’ own fortunes began to decline. In 2013, Forbes reported he was no longer a billionaire. Still, he was a major philanthropist. His charitable donations exceeded $1 billion, according to the obituary on his site. Pickens was also a member of the Giving Pledge, a campaign founded by Bill and Melinda Gates and Warren Buffett to encourage the nation’s wealthiest to eventually donate a majority of their wealth to charity. Pickens was also involved in Republican politics. He was an early backer of George W. Bush’s campaigns as governor of Texas and later president. In the 2016 election cycle, Pickens backed Florida Gov. Jeb Bush, former Hewlett-Packard\n \n (HPQ) CEO Carly Fiorina and Ben Carson before endorsing Trump and eventually agreeing to donate to President Trump’s campaign in 2016 once he became the GOP’s presumptive nominee. Well into his 80s, Pickens was active on social media. In 2016, he famously tweeted that “the first billion is a helluva lot harder” in response to Canadian rapper Drake, who had tweeted about how difficult it was to make his first million dollars. Pickens is survived by five children, 11 grandchildren and “an increasing number of great-grandchildren,” according to the obituary on his site. In the LinkedIn post announcing his retirement, Pickens reflected on his decades-long career in oil. “It has been one hell of a roller coaster ride,” he wrote last year. “I’ve seen oil prices bounce around from $10 a barrel up to $147, down to $26 and now appear to be inching up ever so slowly. I’m ecstatic that I’ve hung on long enough to see it all unfold.” – CNN Business’ Matt Egan contributed to this report.