Yahoo chief executive Marissa Mayer is back to work and with a new chief operating officer in tow.

Mayer — who gave birth on Oct. 1 — tweeted that she was back in the office full-time on Monday, but followed up that announcement with a much bigger one for the company.

A former Googler, Mayer tapped her former company for talent, naming her former colleague Henrique de Castro as the Web firm’s chief operating officer. He will join Yahoo in late January 2013, after finishing his commitments at Google where he is the lead of the company’s worldwide Partner Business Solutions group.

In a statement, Mayer said that she’s excited to have de Castro on board, and particularly highlighted his advertising success.

“His operational experience in Internet advertising and his proven success in structuring and scaling global organizations make him the perfect fit for Yahoo! as we propel the business to its next phase of growth,” Mayer said in a company news release.

De Castro also has a business history in mobile, which Mayer has highlighted as a main focus for Yahoo moving forward. He has also worked at Dell and McKinsey & Company.

In the release, de Castro said that he’s excited to be working with Yahoo as it mounts its efforts at a turnaround.

“The combination of Yahoo!’s unique properties with high quality content, its renewed focus on outstanding user experience and its massive reach bring tremendous value to users, advertisers and partners,” said de Castro in the statement. “This is a pivotal point in Yahoo!’s history, and I believe strongly in the opportunity ahead.

All Things Digital’s Kara Swisher reported that de Castro will have a $600,000 annual base salary, in addition to $36 million in stock grants.

Mayer has been steadily building her team at Yahoo, having named a new chief financial officer, Ken Goldman, late last month.

Related stories:

Marissa Mayer’s plan for Yahoo

Yahoo CEO Marissa Mayer gives birth, will return from leave within weeks

Move by Yahoo’s Marissa Mayer highlights RIM’s troubles