9:06 p.m. | Updated
Marissa Mayer, one of the top executives at Google, will be the next chief of Yahoo, making her one of the most prominent women in Silicon Valley and corporate America.

The appointment of Ms. Mayer is considered a coup for Yahoo, which has struggled in recent years to attract top talent in its battle with competitors. One of the few public faces of Google, Ms. Mayer, 37, has been responsible for the look and feel of some of the search company’s most popular products.

Despite her background, Ms. Mayer — who will be Yahoo’s seventh chief executive in less than five years, two of them interim — will face a daunting challenge.

A pioneering Internet company that helped shape the industry in the 1990s, Yahoo is trying to remain relevant after failing to adapt to changing innovations like sophisticated search technology and social media tools. As Google and Facebook have emerged as Web giants, Yahoo has struggled to create a distinct strategy, even though its audience remains among the largest on the Internet. Now, the company is moving to lay off thousands of employees, in the face of slumping profits and a lackluster stock.

The big question is whether Ms. Mayer — or anyone — can help Yahoo regain its former stature.

“It’s a very interesting departure and a very interesting choice,” said Martin Sorrell, chief executive of WPP, the giant advertising agency. “She comes with a formidable reputation, but we’ll see how she analyzes it all.”

With her appointment as the president and chief executive of Yahoo on Monday, Ms. Mayer joins a shortlist of women in the technology industry to hold the top spot. The elite club includes Meg Whitman, the chief executive of Hewlett-Packard, and Virginia M. Rometty, the head of I.B.M. Another senior woman in Silicon Valley, Sheryl Sandberg, is Facebook’s chief operating officer.

(In a bit of personal news, Ms. Mayer disclosed on Twitter late Monday that she is pregnant.)

The move to Yahoo is an opportunity for Ms. Mayer to step out on her own and claim a bigger stage. Ms. Mayer, an engineer by training whose first job at Google included computer programming, was behind the famously unadorned white search home page and the way users interacted with Gmail, Google News and Google Images. She also sat on Google’s operating committee, part of a small circle of senior executives who had the ear of Google’s co-founders, Larry Page and Sergey Brin.

Perhaps in a sign of grander ambitions, Ms. Mayer joined the board of Wal-Mart Stores in April, her first seat at a public company. She is one of four women on Wal-Mart’s 16-member board.

Still, Ms. Mayer did not have a clear path to the upper echelons of Google.

After years of heading up its search business, Google’s most profitable unit, Ms. Mayer became vice president of the company’s location and local services in late 2010, a group that included Google Maps and more than 1,000 product managers. But the following year, Google promoted another executive, Jeff Huber, to be the senior vice president for local and commerce, putting him one level above Ms. Mayer’s post. Although Google characterized her move as a promotion at the time, some wondered if she would be content with the reorganization.

Ms. Mayer, who was first approached about the job in the middle of June after returning from a trip to China, resigned from Google on Monday afternoon by telephone. She starts at Yahoo on Tuesday and will also join the board.

In an interview, Ms. Mayer said she “had an amazing time at Google,” where she has worked for the last 13 years, but that ultimately “it was a reasonably easy decision” to take the top job at Yahoo. She said Yahoo was “one of the best brands on the Internet.” She recalled that when she started at Google, the company would conduct user surveys and “people didn’t understand the difference between Yahoo and the Internet.”

As she hashes out Yahoo’s strategy, Ms. Mayer said she wanted to focus on the Internet company’s strong franchises, including e-mail, finance and sports. She also hopes to do more with its video broadband and its mobile businesses, tapping into its significant base of users.

For Yahoo, the hope is that Ms. Mayer and her discerning eye will provide some much-needed direction for what has been, as of late, a rudderless ship.

“Yahoo finally has someone who has both business acumen and geek cred at the helm,” said Chris Sacca, a venture capitalist, who previously worked with Ms. Mayer at Google. “She stands for a work hard/play hard, product- and engineering-driven culture, and Yahoo has been missing that for years.”

But she comes to the job with little experience at a companywide level, which could be problematic as she sets a strategic vision.

“I wish I was more excited about it,” said Shar Van Boskirk, an analyst at Forrester Research. “Yahoo has too many products. I fear the challenge, is that by putting a former product person in the C.E.O. role they won’t have somebody who has the ability to create a clear, unified vision and strategy for the Yahoo brand.”

Ms. Mayer also joins a company that has faced significant management upheaval over the last decade as Yahoo has unsuccessfully sought to return to its original glory days. Her predecessors have had little luck revitalizing the former Internet pioneer.

In May, Yahoo’s most recent chief executive, Scott Thompson, resigned after questions emerged about whether he lied about certain academic credentials; he had been on the job for only four months. Yahoo’s board has also been reconstituted, adding three new members, including the activist investor Daniel Loeb of Third Point and Michael Wolf, the longtime media consultant and former chief operating officer of Viacom’s MTV Networks.

For the last several weeks, a half-dozen names had been bandied about to take the top job. But Ms. Mayer’s name was never mentioned. Most analysts said they believed that Yahoo’s board was planning to promote Ross Levinsohn, formerly the company’s head of global media who had taken over as interim chief after Mr. Thompson departed.

“In the last few years, given the turnover, there has been a lack of attention on the user experience,” David Filo, a co-founder of Yahoo, who still works at the company, said in an interview on Monday. “We need to get back to basics.” He said he was very excited that Ms. Mayer agreed to join the company. “It will be a surprise for a lot of people.”