Former 'News of the World' royal editor Clive Goodman is accused of illegally accessing voicemail messages and is also believed to have hacked Prince William 35 times and Prince Harry on nine occasions.
Goodman, 56, denies conspiring to commit misconduct in public office and is one of seven defendants on trial at the Old Bailey who all deny charges against them.
He returned to the Old Bailey court to resume giving evidence after a long period away due to illness.
The court heard how Goodman hacked Kate on Christmas Eve, Christmas Day and Boxing Day in 2005 - more than five years before she married Prince William.
He also hacked her in August 2006, a day before he was arrested over allegations of phone hacking.
Prince William's voicemail was first intercepted in late January 2006, the court heard.
This was the first time that the jury has heard that a mobile belonging to Prince William was hacked.
Goodman denied that he had "forgotten" about targeting the young royals when he said in evidence given earlier in the trial that he had only hacked aides working for the royals.
The former editor has not been in court since the end of March after he was declared unfit to carry on.
The trial continued in his absence and included the testimony of other accused in the case, including former News International CEO Rebekah Brooks.