She is a former Principal Deputy White House Chief of Staff to President Donald Trump and was chief of staff to John F. Kelly during his term as Secretary of Homeland Security. She was confirmed on December 5, 2017, as the Secretary of Homeland Security. Following her appointment, and at the urging of the Trump Administration, Nielsen implemented a controversial practice of separating parents and children accused of crossing over the U.S.–Mexico border illegally.[1][2] Nielsen resigned on April 7, 2019, and Kevin McAleenan, the then U.S. Customs and Border Protection Commissioner, became Acting Secretary.[3]