Kirstjen Michele Nielsen (born May 14, 1972) is an American government official and national security expert. Nielsen was the 6th United States Secretary of Homeland Security from December 6, 2017 to April 10, 2019. Before, she served as Principal Deputy White House Chief of Staff to President Donald Trump from September 6, 2017 to December 5, 2017. She has informally performed the role of Deputy Chief of Staff since John F. Kelly became White House Chief of Staff on July 31, 2017.