The 45-year-old former News International chief executive told the jury in her ongoing trial over allegations of phone hacking that she was unaware the practice was illegal.
Brooks, who was the editor of the tabloid between 2000 and 2003, told the Old Bailey that she "didn't think anybody, me included, knew it was illegal".
"No journalist ever came to me and said we're working on so and so a story but we need to access their voicemail and we need to ask for my sanction to do it. Even though I didn't know it was illegal, I absolutely felt it was in the category of a serious breach of privacy," she said.
Brooks is among seven defendants who deny conspiring to hack phones, to commit misconduct in public office and to cover up evidence to pervert the course of justice.
She told the court that she knew nothing about convicted phone hacker Glenn Mulcaire being tasked to access 13-year-old Milly Dowler's voicemails in 2002 after she was murdered.
She claimed she only became aware Milly's phone had been hacked on July 4, 2011.
Following the revelations, the 168-year-old News of the World was closed later that month.
Asked about her reaction to hearing the schoolgirl's phone had been targeted, she said: "Shock, horror, everything. I just think anyone would think that that was pretty abhorrent, so my reaction was that."
The court was also told details of a contract which agreed to pay Mulcaire 1,769 pounds a week, or 92,000 pounds a year, to supply information.
Asked if she had seen the contract, Brooks told the jury: "No, I didn't."
Asked whether her attention was ever drawn to the contract, which started on September 1, 2001, she said: "Not at the time, no."
Pressed on what she meant by "at the time", she added: "During my editorship."
Mulcaire was jailed in 2007, along with the News of the World's then-royal editor, Clive Goodman, after admitting intercepting voicemails.
Brooks and six others are on trial at London's Central Criminal Court, on charges, including phone hacking, bribery and obstructing a police investigation. All seven have pleaded not guilty.