A Scotland-born mother of four charged with moonlighting as a multimillion-dollar madam has pleaded guilty to promoting prostitution as part of a plea deal.

Anna Gristina, a legal US resident originally from the Scottish Highlands, made the plea at a court hearing in Manhattan, New York.

The judge said she will be sentenced on November 20 to time served and probation, and she could also be deported.

Gristina spent four months in jail before being released on a 250,000 US dollars bond in June.

Her lawyers said she lived on a 12-acre property in Monroe, about 80km north of New York City, and rescued animals and helped abandoned pet pigs find new homes.

But prosecutors accused the 45-year-old of having a roster of wealthy, well-placed clients and boasting of law-enforcement connections during 15 years in a business that made her millions. She had said she was merely starting a dating service.

She had been charged with a single count of promoting prostitution, stemming from a July 2011 tryst which authorities say she arranged involving two women and an undercover officer posing as a client.

Co-defendant Jaynie Baker, a former matchmaking recruiter charged with helping Gristina set up sexual encounters, reached a deal to resolve her case. Baker, 31, is due back in court on October 2.

Gristina was arrested on February 22 as she left a friend's Morgan Stanley office after a fundraising meeting for her business, prosecutors say.

In trying to get the case dismissed, her lawyer, Norman Pattis, wrote that the district attorney's office "vindictively prosecuted her as a result of her failure to co-operate with investigators" during what he called an illegal interrogation.

Gristina said in court papers that investigators shrugged off her requests for a lawyer and told her they would let her go if she gave them information about five men - not named in her filings, but described as a financier, an international banker and a member of a politically connected family, among others.

The DA's office countered in court papers that Gristina "has not produced a shred of evidence of actual vindictiveness".

A grand jury indicted Gristina before her arrest, undermining her argument that she was prosecuted because she did not co-operate, Assistant District Attorneys Elizabeth Roper and Charles Linehan wrote.

Two accused prostitutes and an accused money-launderer also have been arrested in the case.