We all thought advertising veteran Ross Levinsohn was a shoo-in for the top job at Yahoo (YHOO), but boy, did the board have a surprise waiting.

The new CEO is a huge gamble in the form of Marissa Mayer, a 37-year-old executive at Google (GOOG). She has no experience running a public company and has spent nearly all of her professional career at Google, where she was the 20th hire.

Mayer is an engineer by training, and at Google has helped develop Gmail, Google News and Google Images. She lives in a fancy penthouse in the Four Seasons in San Francisco and has been known for her love of cupcakes, The New York Times reports.

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This sounds like another disaster for Yahoo, a company that has churned through four CEOs in four years. Yahoo has one of the worst boards in the business, and the last thing it needs is a young, untested CEO who has never run a company.

Many Yahoo observers had been rooting for Levinsohn, who was filling in as interim CEO after the previous CEO was busted in May for inaccuracies on his resume. Levinsohn was previously the head of global media at Yahoo, and in his short time as chief was already getting the company back in order. He brokered a settlement that ended Yahoo's ridiculous patent lawsuit against Facebook, for example. He hired a different Google exec, Michael Barrett, as chief revenue officer.

But some insiders worried that Levinsohn couldn't be more than just an "ad guy," the Times reports.

What is Yahoo thinking? This strikes me as a Hail Mary pass by the board to find someone who can return Yahoo to its glory days. Mayer is very good at managing products like Gmail and turning them into hits. She ran Google's search products and lately has been helping the company with local business products.

Mayer is a fashionista with a love of Oscar de la Renta clothes and a personal shopper at Bergdorf Goodman. De La Renta told Vogue that she's one of his biggest customers. (Read the Vogue profile here.) But she's also making her way up in the business world, and earlier this year was named to the board at Wal-Mart (WMT).

She may have been passed over at Google when the company named Jeff Huber as its senior vice president of local and commerce, however, Forbes reports. She was not part of Google's core leadership team even after 13 years there, AllThingsD reports.

Yahoo is a media company, after all, but its choice of Mayer shows it wants to refocus on being a product company and a technical company. In a statement announcing the news, Yahoo co-founder David Filo called Mayer a leader in user experience and product design. "I look forward to working with her to enhance Yahoo's product offerings," he added.

Mayer has technical know-how, to be sure. But can she make anything out of a struggling company fighting for its life? She's a very risky choice, but sometimes the biggest risks pay off the most.