Mayer's husband, an investor who focuses on Internet companies, did not mention the name or weight of the baby born Sunday night, but announced online that "Both Marissa and baby are healthy and the family is very excited."
The San Jose (Calif.) Mercury News reported Monday that Mayer, who was hired in July, did not disclose her pregnancy until after she was hired.
Yahoo! said Mayer, 37, would work "remotely and ... return to work as soon as possible, likely in one to two weeks."
In an interview in July, Mayer said she would take "a few weeks" off when her child was born.