Dan Evans has pleaded guilty to phone hacking while working at both newspapers between 2003 and 2010, and is the first journalist to admit hacking for a paper not owned by News of the World proprietor Rupert Murdoch.
After striking a deal with prosecutors, he gave evidence at the trial of former News of the World editors Rebekah Brooks and Andy Coulson and five others. The defendants deny all the charges.
"Principally I was tasked with covering news events, investigations, undercover work, latterly with hacking people's voicemail," Evans said.
He said that in late 2004 later met then-editor Coulson about a job at the News of the World, and told Coulson he could do "stuff with phones", though he did not use the word hacking.
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"I got onto voicemails and interception and I told him I had a lot of commercially sensitive data in my head and how things worked at the Sunday Mirror and I could bring him big exclusive stories cheaply," Evans said.
He said he had accessed voicemails more than 1,000 times in all while at the newspaper.
He was eventually caught after using his work phone, rather than a disposable pay-as-you-go cell phone, when he tried to listen to the voicemails of designer Kelly Hoppen.
She was able to trace the call back to him.
The police investigation spawned by the revelations initially focused on Murdoch's papers, but has spread to take in other companies, including Trinity Mirror PLC, which owns the Sunday Mirror and the Daily Mirror.
In a statement, Trinity Mirror acknowledged Evans' guilty plea and said "it is too soon to know how this matter will progress.