Hagel, 66, has said he did not ask for the Pentagon job but has embraced the opportunity.
"I will do everything in my power to be the kind of leader that you expect and you deserve, also, the kind of leader the country expects and deserves," the Vietnam combat veteran said in 15 minutes of remarks in which he struck a tone of humility.
A two-term Republican senator from Nebraska, Hagel was introduced to his Pentagon audience by a fellow Nebraskan - Sgt. 1st Class John Wirth, of Gordon, Neb., an 11-year Army veteran who served two tours in Afghanistan and one in Iraq.
Wirth was a reminder that Hagel is one of only a few defense secretaries who served in the military's enlisted ranks. He was an Army sergeant in 1967-68 and was wounded in Vietnam. He served in the Senate from 1997-2009 and more recently was chairman of the Atlantic Council, a prominent think tank in Washington.
With a touch of humor, Hagel alluded to his days in the enlisted ranks, where grunts rarely come in contact with four-star generals like Ray Odierno, the Army's top general, who was among the military brass sitting in Hagel's audience.
"He makes me shake a little, being an old Army sergeant," Hagel said with a chuckle.
Hagel said that after taking the oath of office he spent a few minutes walking through an outdoor memorial to victims of those killed at the Pentagon on Sept. 11, 2001. He recalled that he was on Capitol Hill at the moment a hijacked American Airlines jet slammed into the Pentagon not far from the defense secretary's suite, killing 125 people inside the building and all aboard the plane.
He said he "reflected a bit on what happened that day," when nearly 3,000 people were killed in New York City, Washington and in rural Pennsylvania. Quoting the late British leader Winston Churchill, Hagel called the terrorist attack a "jarring gong." It set in motion dynamics "that we are living with today," Hagel said.
Hagel said he felt it important to take time out of his first day as defense secretary to tell the entire workforce that he looks forward to leading in tough times.
"Now I've got to go to work," he said.