[5] Nielsen served during the George W. Bush administration as special assistant to the president and as senior director for prevention, preparedness and response (PPR) at the White House Homeland Security Council.

[8] Nielsen was a senior member of the Resilience Task Force of the Center for Cyber & Homeland Security Committee at George Washington University and served on the Global Risks Report Advisory Board of the World Economic Forum.

[4] Nielsen served as John F. Kelly's chief of staff at the United States Department of Homeland Security (DHS) after he assumed that position on January 20, 2017.

[18] She was questioned about an earlier meeting at the White House in which press reports and Senator Dick Durbin related that the president had used the word "shithole" to describe African countries, as well as disparaging remarks about Haiti.

"[36][37] During the same interview at the Aspen Security Forum when Alexander further asked whether the president has made countering white supremacy a priority, Nielsen replied that he wanted the DHS to prevent "any form of violence" threatening Americans.

Referring to President Trump's response to clashes between the white supremacists and counter-protesters at Unite the Right rally in Charlottesville, Virginia on August 12, 2017, Alexander asked, "But in the comments that are obviously highly publicized when he [President Trump] placed blame in his words on both side, does that make your job harder when [p]resident says things that at least in those communities are viewed as he has got our [white supremacists'] back?"

[41] On October 22, 15 days prior to the 2018 mid-term elections, President Trump met with Nielsen and White House staff and demanded "extreme action" to stop migrants at the southern border.

Later that afternoon at a meeting of top Homeland Security officials, Customs and Border Protections representatives proposed deploying a microwave weapon against approaching migrants.

[42] In January 2019, Nielsen, Acting Attorney General Matthew Whitaker, Commerce Secretary Wilbur Ross, and FBI director Christopher Wray announced 23 criminal charges (including financial fraud, money laundering, conspiracy to defraud the United States, theft of trade secret technology, provided bonus to workers who stole confidential information from companies around the world, wire fraud, obstruction of justice and sanctions violations) against Chinese tech giant Huawei and its CFO Meng Wanzhou.

[43][44][45][46] On May 7, 2018, Secretary Nielsen, despite her objection,[47][48] officially enacted a controversial practice of the Trump administration's policy of separating parents and children accused of crossing over the U.S.–Mexico border illegally.

[58] Nielsen held a press briefing with White House spokesperson Sarah Huckabee Sanders in June 2018 amid growing public outcry about the family separation policy.

[59] She dismissed the suggestion that the administration was using family separations as political leverage to force Congress to support Trump's broader immigration agenda or to deter migrants from coming to the United States.

[59] In doing so, she contradicted comments made earlier by Attorney General Jeff Sessions, Chief of Staff John Kelly and senior adviser Stephen Miller.

[2] In September 2018, The Intercept reported that Nielsen had previously personally authorized the family separation policy after receiving an April 23, 2018, memo by the heads of three federal immigration agencies (U.S.

[72] In a statement Nielsen said that this caravan had acted violently in the past and "I refuse to believe that anyone honestly maintains that attacking law enforcement with rocks and projectiles is acceptable.

[75] In another instance, Jakelin Caal, a 7-year-old girl from Guatemala, died in the custody of U.S. Customs and Border Protection, an agency of the Department of Homeland Security, on December 8, 2018.

[78][79] In a formal statement[80] released on December 26, 2018, Nielsen called the death "deeply concerning and heartbreaking" and cited U.S. immigration system failings for a growing border crisis.

"[80][81][78] In the same statement[80] released on December 26, 2018, Nielsen added, "Smugglers, traffickers, and their own parents put these minors at risk by embarking on the dangerous and arduous journey north.

[...] As a result of bad judicial rulings from activist judges and inaction by Congress, we are seeing a flood of family units and unaccompanied alien children.

"[80] After her arrest in Pattaya, Thailand, on February 25, 2018, Anastasia Vashukevich immediately requested asylum in the United States because she feared for her life if Russia became involved.

[82][83][a] In September 2020, Brian Murphy alleged that Nielsen had politicized the Department of Homeland Security and interfered with DHS intelligence reports to support the views of Stephen Miller and Donald Trump.

[90] In May 2018, The New York Times reported that Nielsen considered resigning after President Trump berated her during a cabinet meeting for what he described as her failure to secure U.S.

[60] She had cut short a week-long trip to Europe where she was going to discuss cybersecurity and terror threats with senior United Kingdom and Swedish government officials.

In a tweet, Nielsen had said that she had agreed to remain in her position until April 10, 2019 "to assist with an orderly transition and ensure that key DHS missions are not impacted.

"[100][101] On April 5, immigration and civil rights groups had urged companies listed in the Fortune 500 not to hire senior Trump administration officials who were involved in planning, carrying out, or defending the separation of migrant children from their parents.

[102] On April 8, a petition aimed at scholars and media figures began circulating, with signers vowing not to "associate myself in any way" with any think tank or university department that employs Nielsen.