Brennan, who grew up in New Jersey, is described by those who know him as a "straight arrow" and man of high morals.
"The word for John is 'intense'," said A.B. "Buzzy" Krongard, a former top CIA official who was once Brennan's boss there. "John's all about commitment."
His long working hours at the CIA and the White House are legendary. Obama, in announcing Brennan's nomination on Monday, quipped: "I'm not sure he's slept in four years."
Brennan pledged, if confirmed, to "make it my mission to ensure that the CIA has the tools it needs to keep our nation safe and that its work always reflects the liberties, the freedoms, and the values that we hold so dear."
Brennan was at the president's side during some of the most significant security incidents during his first term.
The White House last week released a photograph of him briefing Obama on the elementary school massacre in Newtown, Connecticut. He is also visible in an iconic photo of top officials at the White House monitoring, in real time, the U.S. commando raid that killed al Qaeda leader Osama bin Laden in Pakistan in 2011.
Brennan is praised by former CIA officials who have worked with him. "John is a great choice - highly experienced, extremely dedicated, a person of integrity," said John McLaughlin, former acting CIA director.