The appointment of Nielsen likely will mean a continuation of policies followed by Kelly. The retired Marine general won Trump's praise for his tough approach to immigration enforcement during his six months running the department.
Nielsen, who served as Kelly's deputy at Homeland Security before following him to the White House, has experience in disaster response and started a consulting firm that gave her extensive expertise in cybersecurity, a top priority for the department.
In picking Nielsen, Trump passed over potential nominees with close ties to administration officials who favor hard-line efforts to restrict both legal and illegal immigration.
Among the people under consideration, according to a former Homeland Security official, were Kris Kobach, the Kansas secretary of state who is heading Trump’s voter fraud commission and who has been a leader in the immigration-restriction movement, and Rep.
The decision to go with Nielsen indicates that Kelly, by recommending a top aide and a Homeland Security veteran, was able to exert more influence than Stephen Miller, Trump's speechwriter and domestic policy advisor, who has pushed measures to severely restrict immigration.
The choice was greeted with praise and some relief by Homeland Security veterans, who said Nielsen is well prepared to take the job.
"The job is really difficult for anybody. It's part cop, part consoler-in-chief, part cyber-guru and part politician," said Stewart Verdery, a lobbyist and former assistant secretary at Homeland Security. "She's not just an expert in one of them, she's an expert on a lot of them."
Acting Secretary Elaine Duke told her staff about the expected appointment Wednesday, according to the Homeland Security official, who spoke on condition of anonymity to comment on the internal discussions. Trump's decision to choose Nielsen was first reported by Politico.
The sprawling department includes the agencies responsible for policing borders and immigration, a central focus for the Trump administration, including Customs and Border Protection, Immigration and Customs Enforcement and the United States Citizenship and Immigration Services. Nielsen will be the public face for immigration enforcement during the impending fight in Congress over legislation to toughen border security and provide permanent legal status to young people currently covered by the Deferred Action for Childhood Arrivals program, which Trump is ending.
The department also includes the Federal Emergency Management Agency, meaning Nielsen also will assume responsibility for marshaling the government's response to restoring storm-battered Puerto Rico, Houston and Florida. That effort, too, is likely to be a major topic of her confirmation hearings.
"It is extremely important that the DHS secretary stay above the partisan fray and not allow the position to be a political pawn of the White House," he said.
Nielsen served on the White House Homeland Security Council as a special assistant to President Bush and worked on global cybersecurity policy for the United Nations.
She also worked on legislative policy in the Transportation Security Administration, another component of the Homeland Security Department.
3:45 p.m.: This article was updated with reaction to the announcement and additional detail on Nielsen's career.
2:17 p.m.: This article was updated with the announcement of Nielsen's nomination.