Johnson testified before the Senate Homeland Security Committee, which must approve the president's choice. Johnson has held two other jobs that required Senate confirmation and the hearing was generally friendly, The Washington Post reported.
The sprawling department was created after the Sept. 11, 2001, terror attacks and has 240,000 employees. Existing agencies with different missions were thrown into the department, which is responsible for anything from customs and border control to disaster relief.
Johnson told the committee he will improve morale, shown by surveys to be the lowest of any federal department, and fill the 40 percent of senior positions now empty or held by acting executives.
"If confirmed, I pledge to be a champion for every man and woman of the Department of Homeland Security and their families," he said.
Johnson told Sen. Kelly Ayotte, R-N.H., he believes terror suspects captured overseas can be questioned by intelligence agents before they are transferred to the justice system. He said the government needs to be more sensitive to privacy issues as the technology to intrude into people's lives advances.
Sen. John McCain, R-Ariz., pressed Johnson on whether it would be possible to make the border between the United States and Mexico more secure. When Johnson said he could not promise to make the border 90 percent secure until he is more familiar with the problems involved, McCain said he could not support the nomination without that commitment.