Kirstjen Nielsen was sworn in Wednesday as secretary of the Department Homeland Security after she received Senate confirmation the day prior by a vote of 62-37.
Nielsen served at DHS as chief of staff under former head John Kelly in the early days of the Trump administration.
Democrats complained that Nielsen lacks the experience needed to run a major agency with 240,000 employees.
At her confirmation hearing in November, Nielsen maintained she would continue to operate DHS the way Kelly had, including parting with Trump's campaign promise on the proposed border wall.
The key Cabinet position, created after the terror attacks of September 11, 2001, oversees a myriad of agencies tasked with protecting the USA and managing immigration and customs.
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Senate Homeland Security Chairman Ron Johnson, R-Wis., said Nielsen brings valuable, practical experience to DHS. She comes to the job with more cybersecurity and risk management qualifications than any of her predecessors, having founded a risk management firm. Several administration officials say Duke has informed the White House she plans to resign once Nielsen takes over. "She has been working in and around the Department of Homeland Security since its creation". Nielsen said the federal government should adjust its modeling and preparation for dealing with weather events related to climate changes, but refused to acknowledge the alleged human contributions to those changes.
"Throughout her confirmation process, Ms. Nielsen failed to demonstrate that she would provide the steady, experienced leadership - free from political interference from the White House - that the department needs", Sen.
Trump has also ordered DHS to build a wall along the southern border.
Nielsen's appointment had been viewed as a sign of Kelly's tightening grip on the administration since replacing Reince Priebus as Trump's chief of staff in July. He called Nielsen "a willing accomplice, helping to shape and implement this profoundly disturbing and un-American vision of our country".
During the hearing, observed a report in the New York Times, Nielsen responded to Senator Maggie Hassan (D-N.H.) who accused Nielsen of refusing to say that humans are the primary cause of the rise in global temperatures.
But both Kelly and Nielsen have said that a wall on the entire 3,200 kilometres frontier with Mexico would be inappropriate, and that other measures, including electronic monitoring, would be required as well.