The decision by Heritage's board of trustees, announced Tuesday, came after they found "significant and worsening management issues that led to a breakdown of internal communications and cooperation," according to a statement from the board's chair, Thomas Saunders III.
DeMint, once a powerful South Carolina senator before rising to the top spot at Heritage, had been expected
to depart the organization following tension with the board.
"While the organization has seen many successes, Jim DeMint and a handful of his closest advisers failed to resolve these problems," Saunders said. "Heritage has never been about one individual, but rather the power of conservative ideas. Heritage is bigger than any one person."
Heritage will now be steered by Ed Feulner, a former Heritage CEO, who will serve as a temporary replacement.
But DeMint pushed back against the statement issued by the Heritage board. In his own statement, which he circulated late Tuesday night, DeMint said, "The public statement released earlier is puzzling, given that the board of trustees has praised our work for four years and approved performance bonuses for the entire management team each year for a job well done. It also stands in stark contrast to the independent review by the University of Pennsylvania, which publicly recognized advances in Heritage's scholarship, management and integrity over the last four years, and improved Heritage's rankings in virtually every category."
DeMint listed bullet-points highlight of what he considers the successes of his four-year tenure, including playing a key role in Donald Trump's presidential transition and launching the Daily Signal, a conservative news and commentary website, in summer 2014.
"All in all, this is a record of which I am very proud, on behalf of myself, my management team and the many dedicated Heritage staff and members nationwide," he said.
Known for helping fund far-right conservative Republican primary challengers through his Senate Conservatives Fund, DeMint resigned from his Senate seat in 2013 to take the helm at Heritage.
The organization has been closely linked to Trump's campaign and his White House -- including helping craft the list of potential Supreme Court nominees that included recently confirmed Justice Neil Gorsuch.
Its board includes Rebekah Mercer, the GOP financial titan who is close to Trump's organization and holds a stake in Breitbart, the right-wing news outlet that White House chief strategist Steve Bannon ran prior to joining Trump's campaign.
Heritage, which has advanced conservative policy measures since Ronald Reagan's presidency, was deeply involved in Trump's transition effort.
More recently, though, the group bucked Trump and House Speaker Paul Ryan and opposed the GOP plan to repeal and replace Obamacare -- instead pushing for a vote immediately to repeal the Affordable Care Act without replacement provisions in place.