privilege, as well as the greatest responsibility of my professional life," Brennan said. "If confirmed as director, I will make it my mission to ensure that the CIA has the tools it needs to make America safe."
Brennan served as director of the national counter-terrorism centre during George Bush's first term. He was a top intelligence adviser to then-senator Barack Obama during his presidential run and transition to the White House. He is admired in the intelligence community and has a reputation on Capitol Hill for forthrightness and non-partisanship.
If Brennan is confirmed by the Senate, which seems likely, it will represent a reversal of fortunes from 2008, when he was forced to withdraw his name from consideration for director of national intelligence in the face of vocal objections to his support for Bush's torture policy.
Brennan may face a different opposition this time around, although without a public outcry of the kind that met the torture programme. As director of Obama's drone warfare strategy, Brennan has played a key role in using the CIA as a tool of warfare - to the neglect, critics say, of traditional intelligence gathering.
"Brennan is a key architect of that change by stealth of the nature of the CIA," said Scott Horton, a contributing editor on national security to Harper's magazine and a lecturer at Columbia Law School. "I think there's no question in which direction he would push the CIA. It would be towards a greater militarisation and a more prominent role being played in these drone-type operations.
"And the drones are just the beginning of it. We're going to see the CIA take a very prominent role in robotic warfare."
Brennan, 57, was raised just outside of New York City by Irish parents from Co Roscommon. He earned a bachelor's degree in political science from Fordham University, where he studied Arabic. He earned a master's degree in government from the University of Texas at Austin.
In his time at at the CIA, Brennan served as station chief in Riyadh, and as chief of staff to then-CIA director George Tenet. Brennan left government service briefly in 2005, working as a consultant.
Recruited early in the Obama campaign, Brennan soon became one of the candidate's most trusted advisers. Obama is believed to have wanted to give Brennan the job of director of national intelligence in 2008, but just days after the election, Brennan was forced to withdraw. In a letter to the president, Brennan wrote that the challenges ahead of the nation were "too daunting … for there to be any distraction".
He wrote in the letter: "It has been immaterial to the critics that I have been a strong opponent of many of the policies of the Bush administration such as the pre-emptive war in Iraq and coercive interrogation tactics, to include waterboarding. "The fact that I was not involved in the decision-making process for any of these controversial policies and actions has been ignored."
Brennan had repeatedly defended the CIA's use of "enhanced interrogation techniques" in an effort to extract information from terror suspects. "There has been a lot of information that has come out from these interrogation procedures that the agency has in fact used against the real hard-core terrorists," Brennan said in a 2007 interview with CBS News' Harry Smith.
At the same time, Brennan has been a critic of waterboarding and other practices he has referred to as part of the "dark side" of intelligence gathering.
Brennan's reputation grew steadily during the first Obama term, but no moment cast him into the spotlight like the killing of Osama bin Laden on May 2, 2011. Brennan appears in the iconic photograph of the president's advisers as they watch the operation in Pakistan unfold. Brennan went on television the next day to give an account of the operation - an account that turned out to be inaccurate.
Bin Laden, Brennan said, "was engaged in a firefight with those that entered the area of the house he was in. And whether or not he got off any rounds, I quite frankly don't know." It has since emerged that there was no "firefight"; the Navy Seals entered the compound and killed the occupants.
Brennan also said bin Laden had used one of his wives to shield him from bullets - an account that was later discredited. The terrorist leader was shot dead as he stepped to the top of a set of stairs.
Brennan's statements on the current drone programme also have attracted controversy. In a June 2011 speech at Johns Hopkins university, he said that in the prior year "there hasn't been a single collateral death because of the exceptional proficiency, precision of the capabilities we've been able to develop".
The US conducted 113 drone strikes in Pakistan from June 2010 through May 2011, in addition to strikes in Yemen and Somalia. Watchdog groups have named at least six children who were killed by US drone strikes during that period.