Kirstjen Michele Nielsen is an American attorney and government official who served as the Secretary of Homeland Security from 2017 to 2019 during the administration of President Donald Trump. In this role, she oversaw key aspects of immigration enforcement, border security, and disaster response, including the controversial family separation policy at the U.S.-Mexico border. Prior to her appointment as Secretary, Nielsen held several high-level positions in the Trump administration, including principal deputy chief of staff in the White House and chief of staff to John F. Kelly when he was Secretary of Homeland Security. Her tenure was marked by intense scrutiny and criticism from both sides of the political spectrum, particularly regarding the administration's hardline immigration stance.

Nielsen's career in public service began earlier, with roles in the Department of Homeland Security during the George W. Bush administration, where she worked on policy and legislative affairs. After leaving government in 2009, she transitioned to the private sector, serving as vice president at the Homeland Security Advisory Council and later as a managing partner at a consulting firm focused on government and cybersecurity issues. She returned to government service in 2017, bringing her expertise in national security and emergency management. Following her resignation in 2019 amid internal White House tensions, Nielsen has largely stayed out of the public eye but has been involved in advisory roles and speaking engagements on security topics.

Throughout her career, Nielsen has been a key figure in shaping U.S. homeland security policies, with a background that combines legal acumen and executive experience. Her time in the Trump administration positioned her at the center of debates on immigration reform, cybersecurity, and federal response to natural disasters like hurricanes.