After more than 10 years at Google, Mayer joined Yahoo last year and has already helped to raise the company's stock by 86 percent.
Read the full list on Fortune's website.
2. Jack DorseyCo-founder and CEO of Square
He sent the world's first Tweet and has been Twitter's executive chairman since 2011, in addition to heading mobile payment company Square.
3. Mark ZuckerbergCo-founder and CEO of Facebook
One of the most famous young business people in the world, Zuckerberg's reputation looks intact now that Facebook shares have recovered since the company's troubled 2012 initial public offering
4. John ElkannChairman of Fiat
Elkann's grandfather, Gianni Agnelli, chose him for the Italian auto company's board when he was just 21 years old. Since taking the reins after his grandfather's death, Elkann has stabilized Fiat and overseen the company's successful investment in Chrysler.
5. Rob GoldsteinSenior Managing Director of Blackrock
As head of BlackRock Solutions, Goldstein has added 150 clients to the giant asset management firm's institutional business.
See the complete 40 under 40 list here.