When asked by her lawyer whether she had ever approved eavesdropping on voicemails, Brooks answered "no".
She said that as editor of the Rupert Murdoch-owned tabloid between 2000 and 2003 she didn't know phone hacking was against the law, but would have considered it a "serious breach of privacy."
"I don't think anybody, me included, knew it was illegal," Brooks said during her third day of testimony at Britain's phone-hacking trial. "No one, no desk head, no journalist, ever came to me and said 'We're working on so-and-so a story but we need to access their voicemail' or asked my sanction to do it."
Brooks said it was not until July 2011 that she discovered the paper had hacked the phone of 13-year-old Milly Dowler, who was abducted and murdered in 2002.
Brooks said she was on holiday for the period in 2002 when Dowler's phone was hacked. Her deputy and sometime lover, Andy Coulson, was in charge. Coulson, who went on to become Prime Minister David Cameron's communications chief, is also on trial.
Asked by her lawyer, Jonathan Laidlaw, whether she was made aware of the hacking of Dowler's phone during her vacation, Brooks said: "Absolutely not.
Murdoch shut down the 168-year-old News of the World in July 2011 amid public outrage at the hacking of Dowler's phone. Murdoch's media empire has since paid millions to settle invasion-of-privacy lawsuits from dozens of celebrities, politicians and others.
Brooks, Coulson and five others are on trial at London's Central Criminal Court accused of charges including phone hacking, bribery and obstructing a police investigation. All seven have pleaded not guilty.