Mayer was Google's 20th employee and was for many years one of the search giant's most visible and well-known executives. Most recently, Mayer had been in charge of Google's Local, Maps and Location products.

Thompson, who took over as CEO in January, left in May after it surfaced that he didn't have a computer science degree, contradicting his public professional biography. Reports later emerged that his resignation was due instead to a cancer diagnosis.

Thompson was picked after Yahoo's board fired Carol Bartz, who became CEO in 2009 after an illustrious career that included many years as CEO of Autodesk.

Bartz, according to the board, was unable to deliver the expected financial and technological rebirth of the embattled company.

Mayer, who will also be Yahoo's president and hold a seat on the board, will begin on Tuesday, and she'll have her work cut out for her.