Yahoo also lured a slate of expensive journalists to helm a series of new “digital magazines.” Mayer hired David Pogue, The New York Times’s gadget columnist, to be the editor of Yahoo Tech. She personally recruited Joe Zee, the creative director of Elle, telling him to consider Yahoo his “playground.” Mayer’s team asked the former Page Six editor Paula Froelich to run Yahoo Travel; the makeup star Bobbi Brown would oversee Yahoo Beauty. And Yahoo Shine, with its $45 million in yearly revenue, was shot down.

For the NewFronts event in June 2014, Mayer wore a designer dress and introduced a slate of buzzy new shows that she personally approved. Behind the scenes, though, her hands-on strategy was backfiring. “I just think it is a strategic mistake to take on big media where they are strongest,” Jonah Peretti, the C.E.O. of Buzzfeed, wrote me in an email earlier this year, referring to her focus on stars, scripted shows and glossy content. “Especially when there is a huge open space at the intersection of media and tech where it is hard for other big companies to compete.” Couric, meanwhile, had done several interviews with high-profile figures, including former Secretary of Defense Robert Gates, but Yahoo’s users weren’t clicking on the videos. In May, Yahoo Tech had just nine million visitors, placing it in seventh place among competitors, far behind rivals like CNET and Gizmodo. Yahoo Tech would sometimes go weeks without running a single ad. Yahoo Food was 12th in its market. Yahoo’s display advertising revenues dipped 7 percent during the second quarter of 2014.

One of the Yahoo board’s hesitations upon hiring Mayer was her relative lack of experience as a manager. While running search at Google, she oversaw 250 people. Mayer liked to spin her demotion by saying that it left her in charge of more than 1,000 staff members, but a majority of them were contractors. Either way, in her haste to turn around Yahoo, this relative inexperience began to surface. Some on the board had hoped that Ross Levinsohn would stay on as C.O.O., but any hope was jettisoned when Mayer asked him to fly from L.A. for a meeting and then stood him up. Mayer’s refusal to delegate became a sticking point, too. She insisted on personally approving every hire. One executive complained to a friend that Mayer spent as much time deliberating Yahoo’s parking policies as she did strategizing over the sale of its Alibaba stock.

Mayer also had a habit of operating on her own time. Every Monday at 3 p.m. Pacific, she asked her direct reports to gather for a three-hour meeting. Mayer demanded all of her staff across the world join the call, so executives from New York, where it was 6 p.m., and Europe, where it was 11 p.m. or later, would dial in, too. Invariably, Mayer herself would be at least 45 minutes late; some calls were so delayed that Yahoo executives in Europe couldn’t hang up till after 3 a.m. In theory, Mayer kept up with her direct reports through weekly individual meetings. In practice, she often went weeks without seeing them.

This delinquency eventually became a problem outside Yahoo. At a major advertising event in the South of France, Mayer sat for an interview with Martin Sorrell, the C.E.O. of WPP, one of the world’s largest agencies. In front of a filled auditorium, Sorrell asked Mayer why she did not return his emails. Sheryl Sandberg, he said, always got back to him. Later, Mayer was scheduled for dinner with executives from the ad agency IPG. The 8:30 p.m. meal was inconvenient for the firm’s C.E.O., Michael Roth, but he shuffled his calendar so he could accommodate it. Mayer didn’t show up until 10.

At F.Y.I.s, Mayer liked to tell employees that she believed in taking risks and that she was unafraid to admit failure. This philosophy worked well for web products but not for strategic hires. Despite the board’s urging, Mayer opted against vetting Henrique de Castro. As a result, she was unaware that de Castro had a poor reputation among his colleagues in Google’s advertising business. Many had derisively called him the Most Interesting Man in the World, in reference to the satirically fatuous spokesman for Dos Equis beer. De Castro had a tendency to make grand, awkwardly worded pronouncements. He was the inspiration for the Twitter handle @HdCYouKnowMe, which posted tweets that straddled the line between reality and parody: “To incentivize the sales force, you need to hit them with the carrot” and “Product is like snakes . . . slippery — we need someone with a big hammer.” De Castro’s new Yahoo colleagues got a full dose of his strange locution at the company’s annual sales meeting, in early 2013, when he berated his sales force with a rangy, pedantic speech. (De Castro did not respond to requests for comment.)

De Castro’s plan for growing Yahoo revenues focused on user-generated content, like the videos available on YouTube or Instagram. The only problem was that Yahoo did not have access to enough user-generated content to support this plan. (Its attempt to acquire Daily Motion, a YouTube clone, had fallen apart.) As Yahoo’s ad revenues continued to decline, de Castro began to alienate his staff and fellow executives. After one of his direct reports gave a presentation about Yahoo’s business in front of some 40 senior executives, de Castro humiliated the person, saying: “I think your strategy’s more of a fantasy. You make it up. You just make it up.” More important, advertising revenue declined in every quarter since he was hired. Within a year, Mayer had personally taken control of Yahoo’s ad team. De Castro would leave the company in January 2014. For about 15 months of work, he would be paid $109 million.