Kirstjen Nielsen is an American attorney and government official who served as the Secretary of the Department of Homeland Security from December 2017 to April 2019. She was appointed by President Donald Trump and was responsible for overseeing immigration policies, border security, and disaster response efforts in the United States. Before her role as Secretary, Nielsen worked as the Deputy Chief of Staff for John F. Kelly, who was then the Secretary of Homeland Security. She has a background in law and has held various positions in both the public and private sectors, focusing on national security and cybersecurity issues.