If confirmed by the Senate, Nielsen would take the reins at a sprawling department with more than 240,000 employees that is responsible for US border and airport security, immigration policy, disaster response, refugee admissions and other matters.
Nielsen, 45, is a cyber security expert with a considerable resume in homeland security that includes work at the department's Transportation Security Administration and on former Republican President George W. Bush's White House Homeland Security Council.
Nielsen was retired Marine Corps General John Kelly's chief of staff when he was secretary of homeland security during the opening months of Trump's presidency. Kelly brought her to the White House as his deputy when Trump named him chief of staff in July to replace Reince Priebus after only six months on the job.
The nomination requires Senate confirmation.
Nielsen's departure from the White House would mark the latest upheaval in Trump's White House team. She was responsible for carrying out some of Kelly's orders on who gets access to the president. As a result, she has irritated some White House officials who now have limited contact with Trump, according to sources familiar with the situation.
Kelly has sought to bring more order to the chaotic West Wing since replacing Priebus. Trump has welcomed the changes to some extent, although he has privately confided to friends that the limitations on access to the Oval Office sometimes go too far.
Putting Nielsen into the Homeland Security post will allow Trump and Kelly to keep a close eye on the department, but getting her out of the White House could permit some of Kelly's strictness to be relaxed.
The department has been led by an acting secretary, Elaine Duke, since Kelly took the White House post.
Cyber security is one of the primary issues under the Homeland Security Department's portfolio. Nielsen previously worked at a cyber think tank at George Washington University, blocks from the White House, and is considered well-versed in some of the more technical missions at the department, such as sharing cyber-threat information with the private sector.
The department was created after the Sept. 11, 2001, attacks on the United States exposed cracks in the country's homeland security apparatus.
The appointment comes at a busy time for the department, with one of its agencies, the Federal Emergency Management Agency, overseeing disaster relief in hurricane-hit Puerto Rico, Texas and Florida as well as wildfire-ravaged areas of California. The department also is responsible for US border security.
The department is a major player in implementing Trump's aggressive stance towards deporting illegal immigrants, as well as vetting the lower number of refugees Trump has decided to allow into the United States and devising his travel ban on six Muslim-majority nations, North Korea and certain Venezuelans.
"It seems like a low-drama pick. It's a little concerning that she seems to have little background in immigration security and policy, but those individual agencies are in good hands already, and there is a strong core of career managers," said Jessica Vaughan, director of policy studies at the Center for Immigration Studies, which favours more limits on immigration.
Nielsen is not known for hard-line views on immigration like those of White House adviser Stephen Miller.
If confirmed, Nielsen would be the first homeland security secretary to have previously served as a rank-and-file member of the department. Some previous DHS secretaries have been criticized for not possessing enough technical fluency to address cyber threats facing the nation.
She also served as a corporate attorney and a congressional staff member, the White House statement said.
"Kirstjen's a policy wonk at heart, especially when it comes to cyber," Frank Cilluffo, a former senior homeland security official under Bush who worked with Nielsen at George Washington University.
Nielsen would immediately be given the task of helping coordinate the federal response to potential cyber attacks that target elections. US intelligence agencies have concluded that Russia meddled in the 2016 presidential election to try to help Trump win, in part by hacking and releasing emails embarrassing to his Democratic opponent, Hillary Clinton, and through online propaganda.
Russia has denied meddling in the election and Trump has denied any collusion between his campaign and Moscow.
Jeh Johnson, who served as former Democratic President Barack Obama's last homeland secretary chief, designated election systems as critical infrastructure, widening the support the department can provide to states. But the department has clashed with several state officials over how best to cooperate to defend future elections.
Politico first reported the appointment.
(Reporting by Steve Holland in Washington; Additional reporting by Yeganeh Torbati, Dustin Volz, Doina Chiacu and Eric Walsh; Writing by Will Dunham; Editing by James Dalgleish and Peter Cooney)
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