After Kelly became chief of staff to Trump, Nielsen became his deputy chief of staff and special assistant to the president.
Nielsen was Kelly's top aide during his stint as DHS secretary and followed him to the West Wing as his deputy chief of staff when he was appointed in July. Prior to serving the Trump administration, she spent time on the White Hose National Security Council under President George W. Bush and as a senior fellow and member of the Resilience Task Force for George Washington University's Center for Cyber & Homeland Security think tank.
NBC News noted that Nielsen was a "key player" in Trump's travel ban and restrictions on immigration.
North Korea Says Trump 'Lit a Fuse of War'
Kim responded with dire threats and insults of his own, calling President Trump "deranged" and a "dotard". Still, North Korea tested an ICBM that experts said was capable of reaching some parts of the U.S.
The White House says Nielsen has "extensive professional experience in the areas of homeland security policy and strategy, cybersecurity, critical infrastructure and emergency management". But she raised eyebrows when she described the aftermath of Hurricane Maria as a "good news story". And DHS will play a crucial role in continued recovery efforts after a particularly brutal hurricane season that devastated parts of Texas, Florida, Puerto Rico and the U.S. Virgin Islands.
One former Homeland Security official, who asked for anonymity so that the person could speak candidly, says the pick is a controversial one, and that people at the department are already speaking about leaving if the nomination moves forward.
Ridge added, "Kirstjen can hit the ground running and there won't be a learning curve".
Kelly has recently found himself caught in the middle of a deteriorating relationship between Trump and Secretary of State Rex Tillerson, who has, on occasion, expressed frustration with the president and shown a lack of interest in his own staff at the State Department.