Mrs Brooks said she "didn't think anybody, me included, knew it was illegal".
She told the Old Bailey she felt "shock and horror" after she discovered murdered schoolgirl Milly Dowler's phone had been hacked.
Mrs Brooks, one of seven defendants on trial, denies four charges.
She said she was never asked to sanction the accessing of voicemails for a story during her time as editor of the newspaper.
"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'," she said.
"Even though I didn't know it was illegal, I absolutely felt it was in the category of a serious breach of privacy."
Mrs Brooks denies conspiring to hack phones, conspiring to commit misconduct in public office, and conspiring to cover up evidence to pervert the course of justice.
Mrs Brooks said she knew nothing about the tasking of the convicted phone hacker Glenn Mulcaire to access 13-year-old Milly's voicemails in 2002.
She told the court she only became aware Milly's phone had been hacked on 4 July 2011.
The NoW was closed later that month following the revelations.
Asked about her reaction to hearing the schoolgirl's phone had been targeted, the 45-year-old said: "Shock, horror, everything. I just think anyone would think that that was pretty abhorrent, so my reaction was that."
The court heard Milly's mobile phone was hacked between 10 and 12 April, when Mrs Brooks was on holiday in Dubai with her former husband, Ross Kemp.
Her barrister, Jonathan Laidlaw QC, asked: "Was that ever brought to your attention at any point, firstly during your holiday in Dubai?"
"Absolutely not," Mrs Brooks said.
"Or thereafter?" Mr Laidlaw asked, to which she replied "No".
Mr Laidlaw also questioned Mrs Brooks about how often she was on her phone during the Dubai holiday.
He referred to past evidence given by William Hennessy, who was at Dubai at the same time with Dean Keyworth, a friend of Andy Coulson, and who spent some time with Mrs Brooks and Mr Kemp.
He had said he remembered Mrs Brooks walking away to take a call about "the missing Surrey girl".
Mrs Brooks said she did not remember saying that, and that had she referred to Milly, it would have been by name and not as "the missing Surrey girl".
The court heard that Mrs Brooks made four calls to the editor's desk at the NoW on 12 April, sent four texts to Mr Coulson's mobile and received a call from his phone.
Mr Laidlaw asked her if that was an unusual number of calls to be making while on holiday.
She said it was normal given it was a Friday night - when she would usually check to see "how the paper's going" before it went to print.
Taking Mrs Brooks through call records between her phone and the NoW editor's office, Mr Laidlaw also asked her what she would have done had someone told her on the Thursday or Friday that they thought they had a lead on Milly's whereabouts.
"Tell the police," Mrs Brooks said. "I would have assumed probably that they would have told the police before me. If they had not, I would have told them to do so."
The court also heard that a full transcript of a voicemail message mistakenly left on Milly's phone by a recruitment agency had been included in the first and second editions of the NoW that weekend - 14 April 2002.
But references to the voicemail were omitted from the third edition, which was the main selling edition of the paper, the court heard.
Mrs Brooks, who was in Dubai until the Sunday, said she played no part in the change.
She also denied having anything to do with pushing the story from page nine to page 30 of the third edition.
The Old Bailey earlier heard details of a contract which agreed to pay convicted phone hacker Mulcaire Â£1,769 a week, or Â£92,000 a year, to supply information.
Mrs Brooks said she had not seen the contract, which started on September 1, 2001.
Read profiles of the defendants
Mulcaire was jailed in 2007, along with the the NoW's then-royal editor, Clive Goodman, after admitting intercepting voicemails.
Mrs Brooks said a lot of investigators were used during the late 1990s and early 2000s, adding that it was "pretty normal" in Fleet Street.
She said investigators did the "leg work" for journalists, citing an example when the newspaper traced convicted paedophiles living in the community.
She said she had "never heard the name of Glenn Mulcaire" before his arrest.
All seven defendants deny the various charges against them.
Rebekah Brooks has told the phone-hacking trial she did not realise the practice was illegal when she was editor of the News of the World