Linda Yaccarino, NBCUniversal’s advertising chief, was preparing to interview Elon Musk, Twitter’s owner, onstage at a conference last month when she received an email from a peer in the advertising industry.

Rob Norman, a former executive at the ad giant WPP, wanted to know if Ms. Yaccarino had seen the op-ed he wrote after Mr. Musk bought Twitter last year. Mr. Norman’s column discussed the tech billionaire’s amplification of misinformation on Twitter and its chilling effect on advertisers.

Ms. Yaccarino said that she had and that she planned to raise such concerns, Mr. Norman said. But the main focus of her talk with Mr. Musk would be on something else: His efforts to revamp the social network into “Twitter 2.0.”

Now Ms. Yaccarino is set to become the face of Twitter 2.0. Mr. Musk said on Friday that he had selected Ms. Yaccarino, 60, to become the company’s chief executive. Hours earlier, NBCUniversal announced that Ms. Yaccarino was leaving, effective immediately.

“I am excited to welcome Linda Yaccarino as the new C.E.O. of Twitter,” Mr. Musk tweeted. He said she would mainly handle business operations while he would continue working on product design and technology.

In choosing Ms. Yaccarino, Mr. Musk is signaling what his priority is at Twitter: its advertising business, rather than social media know-how. Ms. Yaccarino has been one of Madison Avenue’s power brokers for decades. And Twitter, which makes the bulk of its revenue from ads, has struggled to expand that business, especially after Mr. Musk spooked advertisers last year.

“Linda’s a force,” said Joe Marchese, the former head of ad sales at the Fox Networks Group, who has known Ms. Yaccarino for at least a decade. “She has one of the biggest jobs in advertising, and the ad market is as hard as it’s ever been.”

Yet Ms. Yaccarino will have to do more than contend with Twitter’s advertising woes. The company, which is based in San Francisco, has been severely slimmed down since Mr. Musk slashed 75 percent of its work force and has grappled with gaps in expertise and technical glitches. Twitter is also weighed down by $13 billion in debt that it took on to enable Mr. Musk to buy the company.

Most significantly, Ms. Yaccarino would have to deal with a mercurial and unpredictable boss in Mr. Musk. The 51-year-old billionaire has a track record of firing executives who don’t achieve his goals. He sometimes tweets news about his various companies, which also include the electric carmaker Tesla, without warning. And as Twitter’s owner, Mr. Musk retains absolute power at the company.

Mr. Musk already upended Ms. Yaccarino’s carefully laid plans when he tweeted on Thursday that he had selected a new Twitter chief, though he did not identify her. Ms. Yaccarino, who was in back-to-back rehearsals for NBC’s annual pitch to major advertisers when the tweet went out, hadn’t informed many of her fellow executives that she was planning to leave, four people with knowledge of the matter said.

Lou Paskalis, a longtime ad executive and friend of Ms. Yaccarino, likened her move to Twitter to taking a “step into the lion’s mouth.”

“With her stature in the industry as probably one of the most beloved and trusted people on the revenue side, I question why she would subject herself to that kind of potential reputational risk,” he said.

Mr. Musk and Ms. Yaccarino may be betting that there is plenty of upside with Twitter 2.0. Mr. Musk has laid out ambitious plans for the company, telling employees that it could be worth $250 billion one day and that the platform can be an “everything app,” with features like payments. (He recently said that Twitter is worth $20 billion, down from the $44 billion he paid for it.)

Ms. Yaccarino has already been working on her priorities at Twitter. One person who has spoken with her in recent days said that she is focused on repairing the company’s relationship with Madison Avenue and wooing media companies back to the platform, potentially with partnership deals.

And she and Mr. Musk appear aligned on political issues — such as a more permissive approach toward speech on Twitter — that are central to his vision for the platform, two people familiar with her views said. She is a conservative and a critic of so-called woke discourse, a term used by conservatives to describe elements of left-wing social progressivism they view as censorious, they said.

Former President Donald J. Trump twice appointed Ms. Yaccarino to two-year terms on the President’s Council on Sports, Fitness and Nutrition, where she joined would-be Republican politicians such as Mehmet Oz, the celebrity physician.

Ms. Yaccarino, who did not return requests for comment, grew up with working-class Italian parents in Long Island, New York, including a father who was a police officer. She attended Catholic school. After graduating from Pennsylvania State University in 1985 with a telecommunications degree, she spent nearly 20 years at Turner Entertainment, becoming chief operating officer of advertising sales, marketing and acquisitions before leaving for NBCUniversal in 2011.

At Turner and NBCUniversal, Ms. Yaccarino — who has been said to negotiate like a “velvet hammer” — made a name for herself by helping traditional television hold its ground in advertising in the era of Facebook and Google. Each year, she strode onstage at Radio City Music Hall for the upfront presentations, the glitzy showcases used by television networks to woo Madison Avenue, to persuade marketers to pay a hefty premium over social media rates to advertise on shows like “This Is Us” and “Saturday Night Live.”

But while Ms. Yaccarino has spent years defending TV ad dollars from tech companies and been a fierce critic of Facebook and YouTube, she has also struck partnerships with apps like Snapchat and TikTok and digital outlets like BuzzFeed.

Outside work, Ms. Yaccarino became involved in initiatives including the World Economic Forum’s Taskforce on Future of Work, which she heads. She was also a chair on the board of the Ad Council, a nonprofit, and helped the group raise $60 million in three months early in the pandemic to help counter vaccine hesitancy, making private calls, sending notes and “working every lever that she had,” said Lisa Sherman, the council’s chief executive.

Ms. Yaccarino reached out to Mr. Musk around the time of the Super Bowl in February to discuss a partnership with NBCUniversal and potentially joining Twitter as chief executive, three people familiar with their talks said.

She had previously expressed admiration for Twitter, calling the platform “the single, No. 1 biggest” content distribution partner for NBCUniversal at an ad industry event soon after Mr. Musk took over the company. At the time, she added that she did not plan to “bet against him” and that she believed he could “learn advertising.”

“I think we can teach him,” she said.

This week, Ms. Yaccarino was in attendance when Mr. Musk spoke at an advertising conference in California’s Napa Valley hosted by WPP, three people familiar with the event said.

Ms. Yaccarino would be a rare female chief executive in technology, as top executives like Meta’s Sheryl Sandberg and YouTube’s Susan Wojcicki have recently left their roles. Throughout her career, Ms. Yaccarino has often said that she has been the only woman at the table and has described incidents of bias, such as the time a male supervisor complained in an otherwise flattering performance review about her aggressiveness: “I only wish she would stop using her high heels as a weapon.”

While Ms. Yaccarino is active on Twitter, her habits are sedate compared to Mr. Musk’s, though in recent weeks, she has liked dozens of posts by and about him.

Still, the differences between Mr. Musk and Ms. Yaccarino were clear last month at the media conference in Miami. A polished Ms. Yaccarino came with prepared comments. An unshaven Mr. Musk spent a few moments wrangling his toddler son, X Æ A-12, before joining her and offering sometimes halting answers to her questions.

Ms. Yaccarino returned repeatedly to worries that her industry colleagues have voiced since Mr. Musk took control of Twitter, emphasizing several times that the audience of ad executives was crucial to the company’s financial success.

Mr. Musk said that “there’s legitimate concerns that advertisers have that I want to hear.” He recounted a complaint he had heard from David Zaslav, the chief executive of Warner Bros. Discovery, who was frustrated that he was unable to place ads for “White Lotus,” the hit HBO show, next to discussions of “White Lotus” on Twitter.

The issue has since been fixed, Mr. Musk said.

Ms. Yaccarino answered: “So it’s a new beginning.”

John Koblin contributed reporting from New York.