Six documents taken from an old account belonging to Mr Brennan were published by the anti-secrecy group WikiLeaks.
The CIA said there was "no indication" that anything classified was released.
But the documents do include a draft security clearance application containing personal information.
They appear to all date to 2009, when Mr Brennan was seeking security clearance while applying for a job as a White House counterterrorism adviser.
He was working in the private sector at the time, having already spent 25 years working for the CIA between 1980 and 2005.
A high-school student claimed he was responsible for hacking the AOL account, telling Nthe New York Post he was protesting against US foreign policy.
The boy's Twitter account, which says he is 13, has published redacted images of what appears to be government information.
The documents published by Wikileaks include a draft on national security challenges said to have been created in 2007.
The final, unfinished paragraph is headed "Damaging Leaks of Classified Information".
A 2008 letter about interrogation methods is also included, appearing to be from the vice-chairman of the Senate Select Committee on Intelligence to his fellow board members.
Wikileaks said it would release more documents "over the coming days".
The FBI and the Secret Service are investigating the hacking. The CIA stressed that none of the documents released so far were classified.
"In fact, they appear to be documents that a private citizen with national security interests and expertise would be expected to possess," a statement said.
High-profile leaks including a wealth of diplomatic cables from Wikileaks have dogged the US government in recent years.
The use of private email has become a major issue in recent months for White House hopeful Hillary Clinton, since it emerged that she had operated a private account while secretary of state.
Mr Brennan became director of the CIA in 2013, having spent four years as assistant to the president for homeland security and counterterrorism.
The CIA has condemned the hacking of director John Brennan's personal email account, describing it as a "crime" carried out with "malicious intent".