Icahn is worth about $18 billion, making him the 23rd richest person in the U.S., according to the Bloomberg Billionaires Index. He primarily invests his own fortune rather than relying on money from outsiders, and has investments across industries including health care, energy, metals, auto parts, casinos, telecommunications, real estate and rail cars.

Icahn said he was ending his arrangement with Trump's blessing, "because I did not want partisan bickering about my role to in any way cloud your administration."

Pending decisions regarding AIG's status

Democratic Senators Elizabeth Warren of Massachusetts and Sheldon Whitehouse of Rhode Island sent a letter in July Treasury Secretary Steven Mnuchin, asking whether there was correspondence between the billionaire and administration officials about AIG. The activist investor earlier had pushed for a break up of the insurer in order to help it escape its regulatory tag as "too-big-to-fail," but eased his demands this year. 

"We write to seek assurances that Mr. Icahn has not provided input on or received information" on a pending decision regarding AIG's status as a systemically important financial institution, the senators wrote. Icahn is one of the top five shareholders of AIG.

Prices for biofuel compliance credits known as renewable identification numbers (RINs) have been volatile since Trump's election and Icahn's appointment as special adviser.

Six Democratic senators asked the White House and Environmental Protection Agency administrator Scott Pruitt to detail their interactions with Icahn on the issue. They argued that Icahn's appointment ran afoul of ethics standards and presented a conflict of interest because Icahn hadn't made any obvious effort to separate his business holdings from his broad mandate to address regulations in his advisory role.

The value of Icahn's stake in the Texas refiner increased some by $491 million between Nov. 8 and Aug. 2, to about $1.4 billion, a period running from Trump's election to the day before reports surfaced that the Trump administration was set to reject the bid to relieve refiners of the biofuels burden.

Departure cheered

Icahn's departure was hailed by some of his fiercest critics, some of whom linked the move to the EPA's decision to reject the biofuel policy change the billionaire had advocated.

"Until a combination of public scrutiny and the lobbying of ethanol interests defeated his effort, Icahn appeared poised to leverage his special status with Trump into a regulatory tweak that would save his refining company $200 million annually," said Robert Weissman, president of the watchdog group Public Citizen.

Related: Icahn says AIG presentation inadequate, seeks director slate

"Unfortunately for Icahn, the scheme collapsed. Now that his opportunity for personal profit seems to have vanished, so too has his interest in regulatory policy," Weissman said.

Icahn endorsed Trump for president in September 2015, shortly after the real estate developer and reality television star announced his run, and was frequently mentioned as a potential Treasury secretary, a role he repeatedly said he had no interest in.

Named special adviser in December

Prior to being named a special adviser in December, he'd backed many of Trump's campaign ideas. He supported tax deals to encourage corporations to bring back overseas profits and stay based in the U.S., and backed calls to end the carried interest tax discount for hedge fund and private equity managers.

Icahn, 81, attended Trump's post-election victory party but left in the early hours of the morning to bet about $1 billion on U.S. equities, he told Bloomberg at the time. U.S. stock prices tumbled on news of Trump's election but quickly rebounded, on the way to a string of record highs.

Related: Icahn urges drastic AIG shift, says CEO credibility on the line

Having achieved fame and fortune as a corporate raider in the 1980s, Icahn rebranded himself as an activist investor and outspoken shareholder advocate. In recent years, Icahn has taken stakes in companies including Apple Inc., Hertz Global Holdings Inc., Xerox Corp. and eBay Inc., agitating for shareholder-enriching changes.

A message left for Icahn's spokeswoman wasn't immediately returned.

Icahn, in his letter to Trump, repeatedly diminished his influence over biofuel policy, describing himself as having shared only "limited insights" with the president. "I sincerely regret that because of your extremely busy schedule, as well as my own, I have not had the opportunity to spend nearly as much time as I'd hoped on regulatory issues,"Icahn wrote.