Yaccarino is a highly respected advertising executive who stepped down from NBCUniversal "effective immediately" on Friday as rumors swirled that she would replace the mercurial tycoon as Twitter CEO.
Twitter is struggling to find its footing after Musk's controversial takeover that saw him fire thousands of staff and welcome back far-right and controversial figures on the platform, scaring off its highest paying advertisers.
Musk on Thursday teased Yaccarino's hiring, saying in a tweet that he had hired a woman to replace him as boss of Twitter and its newly named X Corporation parent, but without revealing the name.
Yaccarino's departure from the company that owns NBC, Universal and Telemundo—where she has been since 2011—came a few weeks after she interviewed Musk at a marketing conference in Miami.
Asked by Yaccarino at the time how it's been going since the acquisition, Musk replied: "It's going well...It's entertaining....It's a trainwreck sometimes."
Yaccarino will face an uphill battle trying to lure back major advertisers that are afraid of unwanted associations with conspiracy theorists and far-right content makers who Musk encouraged back to Twitter.
As a leading member of the US media establishment, Yaccarino's hiring blindsided that audience, with her senior role at the World Economic Forum especially singled out for vitriol.
Yaccarino "is a vaccine-pushing, pro mask-wearing, WEF globalist," wrote conservative website Outkick.
In a tweet, Musk said he would remain in charge of design and technology at Twitter, with Yaccarino focusing primarily on business operations and turning Twitter into an "everything app" called X.
Musk's history with the letter X goes back to 1999 when he helped created X.com, an online bank that was bought and later morphed into PayPal.
He has also longed to create a so-called everything app that would be modeled on China's WeChat, which functions as a Twitter-like social media platform as well as offering messaging and mobile payments.
According to NBCUniversal's website, Yaccarino led a team of 2,000 people there and "reengineered the advertising business for the 21st century."
Her job involved nurturing relationships with the world's biggest companies in order to get them to shell out money to advertise on the NBCU family of channels.
"Yaccarino is exactly what Twitter needs to regain advertiser trust and bring back big brands," said Jasmine Engberg, an analyst for Insider Intelligence.
"She will also bring back the professionalism that Twitter has lost under Musk (but) that said, she's still going to have her hands full," she added.
Musk has fully owned Twitter since late October and has repeatedly courted controversy as CEO, sacking most of its staff, readmitting far-right figures to the platform, suspending journalists and charging for previously free services.
With Yaccarino's hiring, he is belatedly fulfilling the promise he made to honor the results of a Twitter poll he posted in December.
In that unscientific survey, a total of 57.5 percent of more than 17 million accounts voted for him to step down.
Wall Street had grown frustrated that Twitter was luring Musk away from his other businesses, and share prices in Tesla rose on news he had found a CEO. (AFP)