Testifying in the trial of two former personal assistants of art dealer Mr Saatchi and his then-wife Nigella Lawson, Sharrine Scholtz said the couple allowed staff to spend company money on designer goods, beauty treatments and leisure classes.
Miss Scholtz, who worked under Mr Saatchi's finance director Rahul Gajjar for four-and-a-half years until 2009, spoke of the stress connected with her job, having to work 19-hour days before signing a compromise agreement and agreeing to "go quietly".
Prosecutor Jane Carpenter claimed Ms Scholtz, who set up an art gallery while still employed by Saatchi, was in court to assist sisters Elisabetta and Francesca Grillo, who deny fraud.
But Ms Scholtz replied: "I have everything to lose by standing up here and nothing to gain."
She said she was "committing career suicide by speaking up" and insisted: "I'm here to speak the truth."
Asked by Anthony Metzer QC, defending Elisabetta, to elaborate on the comment, Ms Scholtz, 29, said: "Anybody who speaks up against the Saatchi gallery is essentially blackmarking themselves from working anywhere in the art world."
Ms Scholtz, who broke down in the dock as she recalled working for Mr Saatchi's Conarco company until she received a sizeable payout to leave, said her role was to log entries relating to company credit card spending by Mr Saatchi's and Miss Lawson's personal assistants, including the Grillo sisters.
The defendants, of Italian descent, are on trial at Isleworth Crown Court, west London, accused of spending £685,000 on credit cards belonging to the celebrity couple to buy designer goods and luxury holidays.
Elisabetta, 41, and her 35-year-old sister, both of Kensington Gardens Square, Bayswater, west London, each deny a single count of committing fraud by using a company credit card for personal gain between January 1 2008 and December 31 last year.
Ms Scholtz said expenses were filed under several types, including "entertainment" and "staff welfare".
Asked by Mr Metzer to elaborate on the credit card expenditure which Ms Scholtz was required to scrutinise, she said: "Entertainment was for restaurants, as far as I can remember, tickets perhaps to events and shows.
"Staff welfare - I cannot really remember what I posted under that. I suppose it would be clothes and things like that.
"(Beauty treatments) would probably go under staff welfare too."
Ms Scholtz, who was 20 when she started working for Mr Saatchi, told the court she "was not aware" of every time staff were allowed to put personal expenses on the company account.
She also said there was no way to tell what was business and what was personal expenditure on the cards.
Under cross-examination from prosecutor Ms Carpenter, Ms Scholtz said: "It wasn't up to me to differentiate between business and personal. I was taught to categorise and put it through."
Asked by Mr Metzer if members of staff were able to spend on designer goods, Ms Scholtz said: "As far as I was aware, they were authorised by Nigella and Saatchi. It would come under staff welfare."
When he asked if she thought there would be "consequences" if she did not sign the compromise agreement, Ms Scholtz replied: "Perhaps I would have been sat there instead of Francesca and Elisabetta."
Asked by Mr Metzer about working conditions at the company, Ms Scholtz said if Mr Saatchi wanted something doing, he wanted it doing "yesterday", causing laughter from the public and press galleries.
In character references read to the court by Mr Metzer, Elisabetta, also known as Lisa, was described as "honest" and "trustworthy".
Jane Winkworth, founder of the French Sole shoe company, employed Elisabetta as a sales assistant in one of her stores for three-and-a-half years from October 2004.
She said Elisabetta was an "exemplary employee" who eventually became an assistant store manager, and said when she heard of the fraud allegations: "I was very shocked because this is totally out of Lisa's character."
Elisabetta currently works as a sales assistant, with her current boss Rose Wakefield, who hired her in November 2012, saying in a statement: "She has never given me any cause to doubt her trustworthiness."
The Grillos' other sisters also provided references, with Michaela saying Elisabetta was "sweet and intelligent" and Anna-Maria describing her and Francesca as being at the heart of their family.
An ex-boyfriend of Elisabetta said she appeared fond of Mr Saatchi and Ms Lawson, adding: "I remember her telling me about the Saatchi-Lawson family only in a positive way."