Special counsel Hampton Dellinger faced President Trump head-on and lost, resigning his post on Thursday rather than fighting his firing from the outside.

His departure marks a major legal victory for Mr. Trump, removing someone who had emerged as a major impediment to the president’s agenda.

Mr. Dellinger led the Office of Special Counsel, which acts as a watchdog for federal employees who face adverse personnel decisions. This week alone, he secured a delay in firing more than 5,000 Agriculture Department employees.

After an appeals court loss Wednesday, he said he was “ending my legal battle.”

“My fight to stay on the job was not for me, but rather for the ideal that OSC should be as Congress intended: an independent watchdog,” he said.

He was appointed by President Biden and took the job a year ago. His tenure was supposed to run for five years.

Mr. Trump fired him in a Feb. 7 notice.

Mr. Dellinger argued that his post was immune to the president’s firing at will. He said he could be ousted only for cause, which the president did not provide.

The White House said Mr. Dellinger was exercising core executive powers and must be subject to removal at the president’s whim. Otherwise, the president could face an opponent from within his administration.

Indeed, that was exactly what happened when Mr. Dellinger challenged the administration’s move to fire probationary employees.

“In short, a fired special counsel is wielding executive power, over the elected executive’s objection, to halt employment decisions made by other executive agencies,” acting Solicitor General Sarah M. Harris told the Supreme Court in arguments last month.

A federal district court had sided with Mr. Dellinger.

Judge Amy Berman Jackson, an Obama appointee, ruled last weekend that the special counsel occupied a unique role in government, with duties to report to the president, Congress and the public.

She said Mr. Trump’s attempt to fire him was “unlawful” and issued an injunction to keep him in office while the legal battle developed.

The U.S. Circuit Court of Appeals for the District of Columbia stayed that decision in an order Wednesday night.

That meant Mr. Dellinger could have continued the challenge to his firing but could not hold the office as the case proceeded.

“I strongly disagree with the circuit court’s decision, but I accept and will abide by it. That’s what Americans do,” he said.

Mr. Dellinger’s case reached the Supreme Court after Judge Jackson issued a restraining order keeping him in his job. The justices demurred at that time.

On Thursday, with Mr. Dellinger giving up his fight, they dismissed the case as moot.

Some legal experts have said his case could be a key test for the justices to decide the extent of Mr. Trump’s firing powers.

Mr. Trump is challenging a 90-year-old precedent that limits the president’s authority to fire leaders of “independent agencies,” including his firings at the National Labor Relations Board and the Merit Systems Protection Board.

Federal judges have blocked both of those firings.

On Thursday, Judge Beryl A. Howell ruled that Mr. Trump exceeded his powers by trying to fire Gwynne A. Wilcox, a member of the NLRB. She accused the president of “a power grab.”

“A president who touts an image of himself as a ‘king’ or a ‘dictator,’ perhaps as his vision of effective leadership, fundamentally misapprehends the role under Article II of the U.S. Constitution,” the judge said.

The Justice Department filed a notice of appeal hours after the ruling. It has also appealed the MSPB ruling.

Ironically, the MSPB and Mr. Dellinger worked together to foil some of the firings of probationary workers. Mr. Dellinger asked for, and the MSPB granted, a 45-day pause in thousands of those firings.

Mr. Dellinger was in the middle of a full investigation to determine whether those firings adhered to the law.