Trump's allies have criticized McGahn for his extensive cooperation with special counsel Robert Mueller, who is investigating the Russian government's efforts to interfere in the 2016 presidential election and possible ties to the Trump campaign.
Trump announced the development on Twitter.
Donald J. Trump
White House Counsel Don McGahn will be leaving his position in the fall, shortly after the confirmation (hopefully) of Judge Brett Kavanaugh to the United States Supreme Court. I have worked with Don for a long time and truly appreciate his service!
8:00 PM - Aug 29, 2018
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McGahn served as Trump's lawyer during the 2016 campaign before assuming the top White House legal post. He has been shepherding Kavanaugh to senators' offices in recent weeks for lengthy introductory meetings ahead of the nominee's confirmation hearings, which start next Tuesday. The White House is hopeful the Senate will confirm Kavanaugh's nomination in time for him to join the high court before its new term starts October 1.
Wednesday's announcement came amid reported tension between Trump and McGahn, who is said to have been interviewed several times by investigators working for Mueller. Reports said McGahn answered questions about many inside-the-White House events related to actions that Trump has taken, although McGahn's lawyer said he did not implicate the president in wrongdoing.
Asked by a reporter Wednesday afternoon whether he was concerned about McGahn's sessions with federal investigators, the president replied, "Not at all," explaining he had approved the discussions.
"We do everything straight. We do everything by the book," Trump added.
Exasperation with Trump's temper prompted McGahn to nickname the president "King Kong," according to a recent article in The New York Times.
"McGahn's relationship with the president has been strained for quite a while due to the ongoing Russia probe," Bradley Moss, a national security lawyer, told VOA.
"His likely successor, Emmet Flood, is far better suited experience-wise to lead the legal response" to the special counsel's requests, said Moss, the deputy executive director of the James Madison Project, a Washington, D.C., organization that promotes government accountability and the reduction of secrecy.
McGahn has been viewed inside the White House and among conservatives as a key member of Trump's team, leading the successful efforts to put like-minded judges on federal benches and cut government regulation.
McGahn "has been very effective at implementing the president's priority of appointing highly qualified judges who have a traditional, modest understanding of their role in our system of government," according to Thomas Jipping, deputy director for legal and judicial studies at the Heritage Foundation, a conservative public policy think tank in Washington.
"That process has a lot of moving parts and political volatility, but Don has stayed on target and kept it moving," Jipping told VOA.
Trump asked McGahn in June 2017 to fire Mueller. According to media reports, McGahn refused and threatened to resign.
McGahn, 50, a former chairman of the Federal Election Commission, will become the latest in a long line of officials to leave the Trump administration, through firings, pressure to resign or voluntary exits.
His departure will come as the White House prepares for a likely onslaught of congressional investigations if the Democrats retake the House of Representatives in the November midterm elections.
"I'll miss him very much," White House counselor Kellyanne Conway told VOA, noting her second-floor West Wing office is "right next to his. We're in the cool kids' corner."
Asked whether McGahn found out from the president's tweet earlier Wednesday that he would soon be leaving his job, Conway did not respond.