Rice's selection was greeted by a muted response from some Republicans who had earlier accused her of being part of an administration cover-up in the Benghazi attacks.
Arizona Sen. John McCain, one of Rice's harshest critics, wrote on Twitter Wednesday that he disagreed with her appointment but would "make every effort" to work with her on important matters. And Tennessee Sen. Bob Corker, the top Republican on the foreign relations committee, said he had spoken with Rice and looked forward "to working with her on shaping important foreign policy and national security issues."
Rice, who first started working for Obama during his 2008 presidential campaign, has a close relationship with the president and many of his advisers. Her long-standing ties to Obama are expected to afford her significant influence within the White House.
The 48-year-old also served in various national security positions during the Clinton administration, including in key roles on peacekeeping and African affairs. Her world view is said to have been shaped by Clinton's decision to not intervene in the Rwandan genocide, a move Rice said later deeply affected her.
Power won the 2003 Pulitzer Prize in general nonfiction for her book "A Problem From Hell: America and the Age of Genocide," which examined U.S. foreign policy toward genocide in the 20th century. She is a graduate of Yale University and Harvard Law School.
According to a biography on the White House website, Power also served as a professor at Harvard's John F. Kennedy School of Government, where she taught courses on U.S. foreign policy, human rights, and extremism.
The White House official said Donilon is expected to stay on the job until early July, after Obama wraps up overseas trips to Europe and Africa, as well as an unusual summit in California later this week with Chinese President Xi Jinping.
Donilon has overseen a foreign policy agenda at the White House that put increased emphasis on the U.S. relationship with Asia. He's also played a key role in the administration's counterterrorism strategy, including the raid that led to the death of Osama bin Laden, and in managing the complex U.S. ties with Russia.