Linda Yaccarino, the chief executive of X and a top lieutenant to its owner, Elon Musk, said on Wednesday that she was leaving the company two years after joining the social media platform.

In a post on X, Ms. Yaccarino, 61, said: “When @elonmusk and I first spoke of his vision for X, I knew it would be the opportunity of a lifetime to carry out the extraordinary mission of this company. I’m immensely grateful to him for entrusting me.”

She did not provide a reason for her departure.

Ms. Yaccarino’s exit caps a tumultuous period at X, which was previously called Twitter and has been remade in Mr. Musk’s image since he bought the platform for $44 billion in 2022. Since then, Mr. Musk has shed three-quarters of the company’s employees, loosened speech restrictions on the platform and wielded X as a political megaphone. Advertisers were at one point spooked by the changes, and the social media company’s ad business declined.

In March, Mr. Musk said he had sold X, which is a privately held company, to xAI, his artificial intelligence start-up, in an unusual arrangement that showed the financial maneuvering inside his business empire. The all-stock deal valued xAI at $80 billion and X at $33 billion, Mr. Musk said. Since then, xAI has been in talks to raise new financing that could value it at as much as $120 billion.

Mr. Musk, who until recently was regularly working in Washington as an adviser to President Trump, has returned to his businesses, which include the electric carmaker Tesla and the rocket company SpaceX. As he and Mr. Trump have tossed criticisms at one another, Mr. Musk has also recently said he was interested in forming a third political party.

Top executives regularly come and go at Mr. Musk’s various companies. One exception is Gwynne Shotwell, the president of SpaceX, who joined Mr. Musk’s rocket company shortly after its founding in 2002.

Managing X has been a challenge, especially with the platform facing constant questions over its content. On Tuesday, Grok, a chatbot created by xAI, shared antisemitic comments on X that caused an outcry. The chatbot praised Hitler, suggested that people with Jewish surnames were more likely to spread online hate and said a Holocaust-like response to hatred against white people would be “effective.” X deleted some of the posts.

Ms. Yaccarino had discussed her plans to leave with X employees earlier this week, before the incident with Grok, three people familiar with the matter said. X and xAI are largely separate, but Grok’s responses are often widely cited — and criticized — across the platform.

While Mr. Musk has been largely unapologetic about offensive content on X, Ms. Yaccarino was in charge of appeasing lawmakers and advertisers. She campaigned on Capitol Hill for several online child safety bills and was involved in litigation against advertisers who pulled spending from X. Ms. Yaccarino has said that more than 96 percent of X’s top brand advertisers have returned to spending money there during her tenure.

“Thank you for your contributions,” Mr. Musk wrote to Ms. Yaccarino in a post on X on Wednesday.

“I think this was an inevitability when X got layered under xAI,” Lou Paskalis, the chief executive of AJL Advisory, an advertising consultancy, and a friend of Ms. Yaccarino’s, said of her exit. “While she got a lot of advertisers back on the platform through her tenacity, they did not return to their previous levels of spending, and that was very unlikely with Elon behaving the way he did.”

Mr. Musk brought Ms. Yaccarino to X in May 2023 to handle the social media company’s business. She grew close with Mr. Musk early that year when she was an executive at NBCUniversal, and when she pledged to keep running ads on Twitter as other advertisers refused to do so. The two began texting, with Mr. Musk ultimately persuading her to run the company.

Upon joining X, Ms. Yaccarino had her work cut out for her. Mr. Musk quickly made changes that tore up much of the good will Twitter had with users, employees and advertisers, including changing speech policies that allowed noxious content to circulate on the platform. Many advertisers left.

Part of Ms. Yaccarino’s mandate, aside from running day-to-day operations at the company, was to repair those broken relationships. The other part of her job has been to manage Mr. Musk. Famously impulsive, the billionaire has frequently made her job more difficult, including using expletives to tell advertisers that he would not change his ways. He has clashed with foreign governments that have requested takedowns of certain social media accounts. Mr. Musk also erased Twitter’s iconic internet brand by renaming the company X.

Ms. Yaccarino has grappled with the challenges, and in public social media posts, maintained an upbeat attitude as cheerleader in chief even as Mr. Musk fired off late-night posts that infuriated many users while also galvanizing his supporters.

X gained a second wind last year when Mr. Trump won the election. Some advertisers, courted by Ms. Yaccarino, returned to the platform partly because of Mr. Musk’s adjacency to the president. Ms. Yaccarino also spearheaded a lawsuit against prominent advertisers and industry trade groups last year, accusing them of colluding to boycott X.

At an employee meeting in May, Ms. Yaccarino discussed X’s merger with xAI and spoke about plans to more closely integrate the two companies’ staff and technical resources.

On Wednesday, one X employee who showed up to work was Matt Madrazo, Ms. Yaccarino’s son, who joined in 2023 to sell ads to political campaigns.

“So that’s awkward,” he said to colleagues upon arriving at X’s New York office, two people familiar with the interaction said. Mr. Madrazo did not return a request for comment.

Ryan Mac contributed reporting from Los Angeles.