Henrique de Castro, previously vice president of Google's worldwide partner business solutions group, will join the company by 22 January, and will report directly to Mayer. He will manage Yahoo's sales, operations, media and business development worldwide, the company said.
Reports surfaced in August that the company was searching for a COO with "turnaround experience."
In a prepared statement, Mayer praised De Castro's experience in operating an online advertising business and in building up a global organisation.
Before joining Google, De Castro managed Dell's Western European sales and business development activities, and before that was a consultant at McKinsey & Co.
De Castro needs a visa to work in the US, and if he has not obtained one by the time he joins Yahoo, he will be based in London and work for the company's UK subsidiary, according to a Yahoo filing with the US Securities and Exchange Commission.
The filing revealed that Yahoo will pay De Castro a salary of US$600,000 (£373,000) and a performance-related bonus worth around $540,000 (£336,000). He will also receive a one-time retention award worth up to $36 million (£22 million) in stock and performance-based stock options and a further $1 million (£621,000) in cash and $20 million (£12 million) in stock to compensate him for the earnings he forfeited by leaving Google.
Mayer herself has only recently joined the company from Google. Yahoo announced Mayer's appointment as CEO on 16 July and she started the next day.
Her predecessor, Scott Thompson, resigned in May after just five months on the job, following a scandal over misrepresentation of his academic qualifications.
Last month she appointed Ken Goldman as CFO to replace Tim Morse, who will leave Yahoo by year-end. Goldman has a long track record as a CFO in the tech industry, having worked most recently at security company Fortinet and before that at Siebel Systems, Excite@Home, Sybase and chip makers Cypress Semiconductor and VLSI Technology, according to Yahoo.
Mayer will reveal some details of her plans for Yahoo during a conference call with financial analysts on Oct. 22, the company has said.
She announced her return to work on Monday with a Twitter message: "My first full day back in the office, and I'm excited to kick it off by announcing my new COO, Henrique de Castro."
Her previous message was a retweet of husband Zachary Bogue's 1 October announcement of the birth of their son: "Baby boy Bogue born last night. Mom (@marissamayer) and baby are doing great - we couldn't be more excited!"