Grant Pizzey, 50, made nearly £20,000 from the tabloid for tips about “notorious” prisoners including Great Train robber Ronnie Biggs, hate preacher Abu Hamza and serial killers Steve Wright and Levi Bellfield.
He also passed on information to journalist Greig Box Turnbull about life at the high security jail which resulted in stories about Easter eggs, a laptop for a terror suspect and higher prices in the prison canteen, the court heard.
Pizzey was on trial at the Old Bailey charged with misconduct in a public office, with his wife Desra Reilly, 48, who was accused of aiding and abetting him.
The couple, from Widecombe Road, Eltham, south east London, denied wrongdoing but were found guilty after the jury deliberated for eight hours and 40 minutes.
The pair shook their heads as the foreman of the jury delivered the majority verdicts.
Using Reilly as the go-between, Pizzey passed confidential information to Mr Box Turnbull between December 1 2005 and January 31 2012.
Reilly first contacted both The Sun and the Daily Mirror in December 2005 with a story about So Solid Crew rapper Dwayne Vincent, stage name Megaman, being found with a mobile phone after he was attacked in Belmarsh.
In a message to The Sun, she wrote: “I have information regarding a security lapse at Belmarsh. Anyone interested?”
Sun journalist James Clothier got back to her, only to be told that the Mirror had been first, the court heard.
Reilly told him that she did not know how much she would be paid by the Mirror and that it would depend on who she dealt with in future.
As well as the “exclusive” about Megaman, she handed over another tip to the Mirror about a smuggled pen gun security scare at Belmarsh, the court heard.
Mr Clothier responded by promising to “top any offer for stories, particularly from the Daily Mirror”, jurors were told.
But Reilly went on to receive cheques and transfers to her bank account from the Mirror on 47 separate occasions.
Prosecutor Julian Christopher QC said some were for stories while others were to keep the source “on side” and encourage more tips in the future.
The relationship developed to the point where Mr Box Turnbull could use Pizzey to confirm information he had heard elsewhere, such as whether James Bulger killer Jon Venables was at Belmarsh.
Pizzey, who worked as a prison officer at Belmarsh in south east London from 2000, knew he was not allowed to speak to the press, which is why he used Reilly, a railway cafe worker, as the contact.
When the couple were arrested in July 2012, they declined to answer questions in police interview.
The case hinged on whether or not Reilly alone had made money by passing on information she picked up from “casual conversations” with her partner.
Giving evidence, Reilly told jurors that she kept her contact with Mr Box Turnbull a secret from Pizzey after he “poo-pooed” the idea of going to the press.
She said that Pizzey would regularly “let off steam” and regale her with stories about the famous inmates.
The mother-of-four regarded Mr Box Turnbull as a friend who was genuinely interested in her life, but conceded: “Obviously he was just trying to get more information out of me in the end.”
Seeing her stories printed in the Mirror gave the waitress “excitement”.
She said: “When I got the first money, it was like a little bit extra for a couple of sentences and I thought there cannot be much wrong in doing that. It was something on the side. It was excitement in my life that I had never had before.”
Judge Richard Marks refused the couple bail and remanded them in custody until sentencing next Friday.
Afterwards, Detective Superintendent Larry Smith, from Operation Elveden, said: “When public officials behave in this way, they breach the trust and confidence placed in them by the public to act with honesty and integrity.
“In this case the evidence presented to the court showed that Pizzey was aware that in his role as a prisoner officer, selling confidential information obtained in the course of his duties was wrong.
“Using Reilly, he attempted to mitigate this by appointing her as the go-between. Their actions damaged the public trust and merit criminal sanction.”