Nielsen currently serves as the White House principal deputy chief of staff under Kelly, who left the job as DHS secretary to become White House chief of staff in July.
Before Kelly's transition to the White House, Nielsen served as his chief of staff in the DHS. She moved with him to the White House.
"Ms. Nielsen has extensive professional experience in the areas of homeland security policy and strategy, cybersecurity, critical infrastructure, and emergency management," the White House said in a news release announcing her nomination.
She previously worked in DHS as a senior legislative policy director for Transportation and Security Administration under President George W. Bush. Under the Bush administration, she also served as special assistant to the president and senior director for prevention, preparedness and response on the White House Homeland Security Council.