WASHINGTON — Kayleigh McEnany stepped on the podium in the White House Briefing Room on Friday, 417 days since a press secretary for President Trump last held a news briefing, with a promise to the sparse group of journalists allowed in the room under the current social distancing guidelines.

“I will never lie to you, you have my word on that,” she said.

Ms. McEnany, the president’s fourth press secretary, found that vow almost immediately tested.

Discussing the sexual harassment accusation by a former staff member against Joseph R. Biden Jr., the president’s likely general election opponent, she said the president had “always told the truth on these issues,” ignoring Mr. Trump’s evasions and misrepresentations in responding to dozens of accusations against him. She claimed it was unfair to bring up issues from “four years ago that were asked and answered.”

In reality, a new accusation of rape was raised against the president as recently as last year.

In a reference to the special counsel Robert S. Mueller III, she claimed that his investigation of Mr. Trump ended with “$40 million of taxpayer money being lost in the complete and total exoneration of President Trump.” In fact, the cost of the report was about $32 million, according to the Justice Department, and its conclusion was not a “total exoneration” of the president.

She defended Michael T. Flynn, the former national security adviser who pleaded guilty to lying to the F.B.I., whom the president is trying to rehabilitate, by misquoting and mischaracterizing newly released notes showing the way F.B.I. officials debated how to deal with Mr. Flynn and figure out the goals of an interview.

Ms. McEnany claimed one of the notes said, “Quote, ‘We need to get Flynn to lie,’ quote, and ‘get him fired.’” In fact, the note said, “What’s our goal? Truth/Admission or to get him to lie, so we can prosecute him or get him fired?”

And she came prepared with a notebook full of talking points, referring to it as she reiterated Mr. Trump’s blame-defecting attacks on China and the World Health Organization for allegedly contributing to the worldwide spread of the coronavirus.

Over all, Ms. McEnany’s confident performance was less combative than those of one of her predecessors, Sarah Huckabee Sanders.

And her first go-round, while delivering its own share of falsehoods, did not create the same kind of disastrous first impression that Sean Spicer, the president’s first White House press secretary, did when he claimed, falsely, that Mr. Trump’s inauguration crowd was the “largest audience to ever witness an inauguration, period, both in person and around the globe.”

It also made her more accessible than her immediate predecessor, Stephanie Grisham, who finished her brief tenure never having once briefed the news media.

By Ms. McEnany’s own definition of what she said she was there to do for the president, she was a success. “I’m consistently with him absorbing his thinking, and it is my mission to bring you the mind-set of the president, deliver those facts,” Ms. McEnany said.

Ms. McEnany, a Harvard Law School graduate whose last job was working as a spokeswoman for the Trump campaign, pitched herself as part of a new guard in the West Wing. She said she — along with Alyssa Farah, the communications director, and Ben Williamson, another communications adviser who joined the White House as part of a reorganization instituted by Mark Meadows, the new chief of staff — is committed to fostering a new era of transparency with the news media.

But that is a lofty goal three and a half years into the Trump presidency, as Mr. Trump has made it clear that the job of communicating for him is almost impossible, and that trying to undermine the credibility of the news media is central to his re-election efforts.

Ms. McEnany’s posture toward the news media on Friday was friendlier than she has been so far in the job. “Leave it to the media to irresponsibly take President Trump out of context and run with negative headlines,” Ms. McEnany said last week, after Mr. Trump suggested that disinfectants could potentially combat the coronavirus.

On Twitter, she harangued a reporter who referred to Mr. Trump online simply by his last name. “To you, he’s not Trump, he’s PRESIDENT Trump!” Ms. McEnany tweeted last month.

Before her briefing debut, Ms. McEnany spent days preparing, participating in murder board sessions with other members of the press and communications staff, and receiving briefings from economic experts and doctors, officials said.

But administration officials said Ms. McEnany was not expected to hold a daily press briefing, and will conduct briefings interspersed with Mr. Trump’s own sessions with reporters.

The president in recent weeks has complained to aides that he “wants his people out there” more defending himself and his administration’s response to the coronavirus, and the attempt to resuscitate the briefing, officials said, was partly a reaction to that wish.

A bipartisan group of former White House press secretaries, Foreign Service and military officials this year signed an open letter advocating the return of the daily briefing — titled, “Why America needs to hear from its government,” and arguing that “the public has a right to know what its government is doing, and the government has a duty to explain what it is doing.”

But critics said there was a marked difference between holding a daily press briefing and doing it on days when you think it will be to your advantage.

“If there was ever a real news situation, and we didn’t brief, that would have been significant,” said Dan Pfeiffer, a former communications director under President Barack Obama. “When you get to pick the days you brief, it’s not the same thing.”

Mr. Pfeiffer, who had his own problems with reporters, said despite the subdued tone of Ms. McEnany’s first outing with the news media, he was no longer willing to give the Trump administration the benefit of the doubt, given its long track record of dealing in falsehoods.

“The Trump White House is not putting the briefing back on in order to inform the public,” he said. “They’re doing it to create a gladiator style show where the new press secretary will bash the press and Trump will fire up his base.”