The activist investor tweeted Friday afternoon that he had decided to accept a Trump offer for Treasury secretary. Still, he noted that "the methods of electing our corporate and political leaders have become completely dysfunctional."
A representative for Icahn did not immediately respond to a request for clarification.
In a June public statement, Icahn wrote that he was "flattered but do not get up early enough in the morning to accept this opportunity," following Trump floating his name during an MSNBC interview.
Trump complained at that time that America does not have the "best or brightest" negotiators working on trade deals and said he would turn to Wall Street for extra help. Trump also named former General Electric CEO Jack Welch and investor Henry Kravis as other advisors if he were to win an election.
—Reuters contributed to this report.