Though the White House confirmed her nomination, the president gave an on-camera announcement Thursday afternoon.
"During [White House chief of staff] John Kelly's time at DHS, right by his side was Kirstjen, the chief of staff for the department," Trump said. "He would be the first to tell you that she was critical to his success.
"There, she quickly reinforced a sterling reputation as a dedicated leader whose No. 1 priority is always the safety and security of our country and our citizens, not politics of ideology. That sets her apart."
When Kelly, the former DHS secretary, moved into his new role as White House chief of staff, Nielsen came with him to serve as his deputy.
Prior to the Trump administration, Nielsen 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.
"At DHS, Kirstjen was integral to the progress we have made in controlling our borders, confronting gang violence, preparing for disasters and protecting aviation security," Trump said.
As DHS secretary, Nielsen will be expected to push Trump's travel ban efforts as well as his promises on immigration, including the construction of a wall at the U.S.-Mexican border. DHS also is responsible for recovery in the wake of multiple severe hurricanes this summer in Texas, Louisiana, Florida, Puerto Rico and the U.S. Virgin Islands.
"In recent weeks communities across our nation have been struck by natural disasters of historic proportions," Nielsen said Thursday. "Americans have seen the devotion of this department and the extraordinary team at DHS, who along with our federal, state, local, and territorial partners have been working around the clock to provide supplies and save lives."
She has a reputation as an expert on cybersecurity matters, and if confirmed, she would be the first former DHS employee to run the department.
"There will be no on-the-job training for Kirstjen," Trump said. "She is ready on Day 1."