The president praised Hagel as "the leader that our troops deserve" in announcing his choice for the new secretary of defense.
"Chuck Hagel's leadership of our military would be historic. He'd be the first person of enlisted rank to serve as secretary of defense. One of the few secretaries who have been wounded in war and the first Vietnam veteran to lead the department," Mr. Obama said.
Hagel, in brief remarks, thanked Obama "for this opportunity to serve this country again, especially its men and women in uniform. ... These are people who give so much to this nation every day."
But the former Republian senator has already come under fire from congressional lawmakers who say he is anti-Israel and soft on Iran.
"This is an in your face nomination by the president to all of us who are supportive of Israel," Sen. Lindsey Graham, R-S.C., told CNN on Sunday. "I don't know what his management experience is regarding the Pentagon -- little, if any, so I think it's an incredibly controversial choice."
Top Senate Democrats tell ABC News there is no guarantee Hagel will win confirmation and that, as of right now, there are enough Democratic senators with serious concerns about Hagel to put him below 50 votes.
In nominating Brennan as head of the CIA, Mr. Obama hailed him as one of the nation's "most skilled and respected intelligence professionals."
Brennan, a 25-year CIA veteran, was deeply involved in the planning of the 2011 raid that killed Osama Bin Laden.
"People here in the White House work hard, but John is legendary even in the White House for working hard," the president said. "He is one of the hardest working public servants I've ever seen."
"I'm not sure he's slept in four years," Mr. Obama joked.
Brennan withdrew from consideration for the spy agency's top job in 2008 amid questions about his connection to harsh interrogation techniques used during the George W. Bush administration. He denied any involvement, but still withdrew his name from consideration.
If confirmed, Brennan will succeed David Petraeus, who resigned in November after admitting to an affair with his biographer.
The Associated Press contributed to this report.