Loloahi Tapui is a former cleaner to Baroness Scotland
The attorney general's ex-housekeeper lied to her about her immigration status in the UK, a court heard.
Loloahi Tapui, 27, told Baroness Scotland she had nothing to worry about, but her visa had expired by four years, Southwark Crown Court was told.
Ms Tapui admits possessing a passport with a counterfeit visa stamp but denies using it to establish facts about herself and earn money.
She has also admitted lying to her employer about mislaying her passport.
The passport was later found by police on a bookshelf in her west London home, the court also heard.
The false passport stamp showed a date of issue of 12 September 2005 and indicated she had been granted leave to remain in the UK on certain conditions until 12 September 2008.
And the court also heard Ms Tapui's passport clearly showed she had overstayed her leave to remain in the UK by four years.
Baroness Scotland, the chief law officer for England and Wales, appeared as a prosecution witness on Tuesday and asked to be referred to as Patricia Mawhinney.
Asked why she had failed to keep copies of documents which purported to show Ms Tapaui's right to work in the UK, she said: "It's something I bitterly regret now.
"Frankly, I believed her."
She continued: "I'd had a pretty tough couple of months, a number of bereavements in my family. It came at a difficult time."
She interviewed Ms Tapui for the job on 23 January 2009.
She told the court she had only told Ms Tapui she was a lawyer, and had wanted to make sure she "understood how important honesty was to me and my family".
"I told her that it was very important that whoever worked for me, as I was a lawyer, should be entitled to work in this country," she said.
Baroness Scotland also told the jury when Ms Tapui returned to her home in Chiswick the next day, with the documents purportedly showing her entitlement to work in the UK, Ms Tapui told her: "I know who you are."
The court heard how Ms Tapui's husband Alex Zivancevic, also a lawyer, had recognised the name Patricia Mawhinney and said her potential employer was the attorney general.
Baroness Scotland has appeared in court as a witness
Baroness Scotland told the court: "She said, 'Don't worry, we understand your need for security and your need to do everything correctly'."
She later told the court: "I thought this woman was married to a member of the legal profession.
"It never crossed my mind that a lawyer in this country would be married to an illegal immigrant and then pass her off as a cleaner to the attorney general.
"You would need to be brain dead to do something like that."
When asked about Ms Tapui's documents, Baroness Scotland said: "Everything she showed me tended to verify the information that she had given."
Ms Tapui was asked to present her passport, her national insurance documents, her P45, any documents which showed her entitlement to work in the UK and even her marriage certificate to Baroness Scotland.
The attorney general was fined £5,000 in September last year for illegally employing Ms Tapui.
Employers can defend themselves against such penalties if they keep copies of the documents provided, which can help to prove they did not realise the employee was illegal.
Under cross examination by Christopher Hehir, defending Tapui, Baroness Scotland insisted she had checked all of Tapui's documents, but admitted that she had made a mistake by not taking copies of them.
She said: "I made a mistake for which I am very sorry. I'm so sorry about this. I can't tell you how sorry I am.
"But I'm afraid it still doesn't change the fact that she asserted she was lawfully entitled to be in this country and lawfully available for work. She was not. She lied to me."
Asked if the case had caused her any political embarrassment, Baroness Scotland said: "It's caused me considerable pain, but it's caused my family more.
"What she has done has been quite extraordinary. To have lied as she has lied and to damage someone who has tried to be kind to you is quite a difficult thing to accept.
"I was actually really hurt and did ask her how she could do this to me and that's when she said she was sorry. She wasn't so sorry she didn't do it."
Duncan Penny, prosecuting, told the jury: "When the defendant offered herself for employment in January 2009 the Crown alleges that she acted dishonestly, making a false representation that she was entitled to work in the UK.
"When her true status came to light and she was confronted about her status by Baroness Scotland, the defendant admitted she lied during the application process about the whereabouts of her passport."
The trial is expected to last three to four days.