WASHINGTON — President Trump surprised Donald F. McGahn II, the White House counsel, on Wednesday by abruptly announcing that Mr. McGahn will be leaving his job this fall, effectively forcing the long-anticipated exit of a top adviser who has cooperated extensively in the investigation into Russian election interference.

The president made the declaration on Twitter without first informing Mr. McGahn, according to people close to both men. It came 11 days after The New York Times reported the degree to which Mr. McGahn — who was by Mr. Trump’s side at major moments as the president sought to keep control of the Russia inquiry — has emerged as a key witness in the investigation. Over the past nine months Mr. McGahn has given 30 hours of testimony in at least three voluntary interviews.

Mr. McGahn’s departure leaves the White House without one of the few senior advisers who have been willing to push back against Mr. Trump. It also raised the prospect of further West Wing exits, particularly in the White House Counsel’s Office, where Mr. McGahn has had a loyal staff, with several people staying in their jobs out of devotion to him.

Mr. McGahn had long discussed his intention to leave at some point, but the way Mr. Trump blindsided him with a tweet underscored how dysfunctional the relationship had become, even as the Russia inquiry appears to be reaching a critical phase, and the legal scrutiny is mounting for the president.

Mr. Trump’s decision did not appear to be a direct response to the disclosure that Mr. McGahn had been cooperating closely with the special counsel, Robert S. Mueller III — a fact his team was aware of — though its timing inevitably led to questions about whether the president was cutting loose an aide who he believed had provided damaging information about him.

[The White House counsel, Donald F. McGahn II, has cooperated extensively in the Mueller inquiry.]

But Wednesday afternoon at the White House, Mr. Trump praised Mr. McGahn and said he had nothing to fear about what his counsel had told Mr. Mueller, even as he appeared to confirm that he was not completely aware what that was.

“I don’t have to be aware,” he said. “We do everything straight. We do everything by the book. And Don is an excellent guy.”

In his tweet Wednesday morning, Mr. Trump said Mr. McGahn would leave after the Senate votes on the confirmation of Judge Brett M. Kavanaugh to fill the vacant seat on the Supreme Court, the culmination of a quiet but intensive effort he has directed to remake the federal courts by installing scores of conservative judges.

“I have worked with Don for a long time and truly appreciate his service!” Mr. Trump said in the Twitter post.

But the relationship between the president and Mr. McGahn has been rocky since he failed to stop the attorney general, Jeff Sessions, from recusing himself a year ago from the Justice Department’s investigation. The two men also clashed again last June after Mr. Trump asked Mr. McGahn to fire Mr. Mueller. Mr. Trump ultimately backed down after Mr. McGahn threatened to resign rather than carry out the president’s directive.

Mr. McGahn’s departure has been rumored for months, and he had told Mr. Trump earlier this year that he planned to step down soon but had not settled on a date. At the time, Mr. Trump told Mr. McGahn he was reluctant to let him go. But the two men have not discussed the matter recently.

The president’s tweet was precipitated by a report on theAxios website that Mr. McGahn planned to leave after the confirmation of Judge Kavanaugh. Hearings on the nomination will begin next week.

Mr. Trump had grown tired of seeing reports that Mr. McGahn might leave, according to people familiar with his thinking, and decided to take away any wiggle room he might have. Allies of Mr. McGahn said on Wednesday that he believed the story was planted by his critics to force the president’s hand and hasten the timeline of announcing his departure.

Among those critics have been Mr. Trump’s daughter Ivanka Trump and son-in-law, Jared Kushner. Ms. Trump complained bitterly to her father about The Times report this month, which detailed how some in the White House were unaware of the extent of Mr. McGahn’s cooperation with Mr. Mueller, according to a person briefed on the discussion.

One person close to Ms. Trump insisted that she had not criticized Mr. McGahn over his appearance before Mr. Mueller and that she had not played a role in his departure. Mr. Trump himself weighed in on Thursday morning on Twitter.

Mr. Trump and his White House counsel had already grown distant, with the president bristling at being advised not to take actions that could draw legal scrutiny, and Mr. McGahn becoming increasingly weary of serving a client who often refused to listen to legal reasoning.

Mr. McGahn had taken to telling people that a day without a summons to the Oval Office was a good day, and he preferred to spend as much time as possible in his upstairs corner office in the West Wing next to the presidential counselor Kellyanne Conway — like him a New Jersey native — which they called the “New Jersey corner.”

Within the White House, Mr. McGahn was seen as the protector of presidential institutions and someone not afraid to fight the president, and had several epic screaming matches to show for it.

Mr. Trump often griped that he wanted to get rid of Mr. McGahn, but the president never seemed willing to follow through on it. The president asked Rob Porter, then the staff secretary, several times last year if he would be willing to take over for Mr. McGahn, an idea supported by several of his aides and his children. But Mr. Porter told the president that he did not believe he was qualified for the role, felt it was the wrong fit for him and preferred focusing on policy, those briefed on the discussions said. He has since left the White House amid accusations of spousal abuse.

Mr. Trump often blamed Mr. McGahn for the cloud the special counsel’s investigation had cast over the White House.

He said Mr. McGahn should have done more to stop Mr. Sessions from recusing himself from the investigation, the decision Mr. Trump believes cleared the way for Mr. Mueller’s appointment in May 2017. But there was one major decision Mr. McGahn did try to stop, and failed: firing the F.B.I. director, James B. Comey.

Mr. McGahn, who was the top lawyer for the Trump campaign, forged a strong bond with Republican congressional leaders, who regarded him as a rare island of political sense in a sea of White House officials whom they viewed as overly dramatic and politically inept. He had remained in the job, according to people who have spoken with him, in part at the urging of Senator Mitch McConnell of Kentucky, the majority leader, as well as to help execute a strategy he spearheaded to appoint conservative judges.

Mr. McGahn had tried to lay the groundwork for his resignation by persuading Mr. Trump to hire Emmet T. Flood — who was part of the legal team that represented President Bill Clinton in his impeachment proceedings — as his lead White House lawyer dealing with the special counsel inquiry, to position Mr. Flood to then succeed him, according to people close to the discussions.

Still, some Republicans reacted to the news of his impending departure with alarm. In a tweet, Senator Charles E. Grassley of Iowa, the chairman of the Senate Judiciary Committee, pleaded with Mr. Trump not to let him leave the White House.

But some members of conservative legal circles in which Mr. McGahn has worked suggested that he had grown frustrated with serving as the top lawyer in a White House that has drawn more than the usual share of legal scrutiny.

George T. Conway III, who withdrew last year as Mr. Trump’s choice for a top post in the Justice Department and is Ms. Conway’s husband, responded to Mr. Grassley by tweeting, “remember the eighth amendment, senator.” It was a reference to the prohibition in the Constitution against cruel and unusual punishment.