She entered her plea during a brief appearance at U.S. District Court in Manhattan.
As part of the plea bargain, Lewis is obligated to turn over records, testify before the grand jury, if asked, and answer any questions investigators may have about her role in arranging dates between Emperor's Club prostitutes and the agency's deep-pocketed clients.
Her agreement to cooperate was revealed in court papers filed by prosecutors, then reluctantly confirmed by her lawyer, Marc Agnifilo, after a federal magistrate turned down his request to have the records sealed.
Agnifilo said his client has yet to be asked to appear before the grand jury. He added that prosecutors have not disclosed whether the probe's next target is Spitzer, who, so far, has not been charged.
Asked why his client had decided to plead guilty, Agnifilo said Lewis, who majored in English at the University of Virginia and has never been in trouble with the law before, just wants to put the case behind her.
"She's basically a very good person. Sitting at a defense table in a federal courthouse is the last place she imagined she'd be," he said. "I have no doubt she'll never be in trouble again."
Lewis, 32, entered her plea calmly while her mother and sister looked on, then left the courthouse without speaking to reporters. Under federal sentencing guidelines she could face around 16 months in prison or less depending on the level of her cooperation with prosecutors, her lawyer said.
Her sentencing was tentatively scheduled for Aug. 6.
The Emperor's Club investigation began last year when banks flagged suspicious cash transfers to companies set up to disguise payments by the ring's clients. Some of the transfers were traced to Spitzer.
The FBI disclosed in court papers that agents secretly recorded conversations between Lewis and Spitzer about a Feb. 13 tryst in Washington with a prostitute named "Kristen." The former governor, identified in court papers only as Client 9, allegedly paid $4,300 for the night, with some of the money to be used as a credit against future encounters.
Spitzer, who is married and has three teenage daughters, abruptly resigned March 12, just days after his role in the case became public.
The real name of the woman identified as Kristen is Ashley Alexandra Dupre, a 23-year-old high school dropout from Beachwood, N.J. She has not been charged.
The other defendants are Mark Brener, 62, of Cliffside Park, N.J., who is accused of running the ring; Cecil Suwal, 23, who lives with Brener; and Tanya Hollander, 36, of Rhinebeck, N.Y., who authorities said also worked as a booking agent for prostitutes.
Investigators say the ring charged between $1,000 and $5,500 per hour for dozens of prostitutes in New York, Washington, Los Angeles, Miami, London and Paris.
Prosecutors have refused to say whether they are contemplating charges against Spitzer, who was New York's attorney general for eight years before he was elected governor.
As attorney general, he built a reputation as a crusader against shady practices and overly generous compensation on Wall Street and, when he was elected, pledged to bring higher ethical standards to Albany. His cases as attorney general included prosecutions of prostitution rings.
After his role in the call girl ring became public, Spitzer - with his wife, Silda, by his side - apologized at a news conference without expressly acknowledging that he had visited prostitutes.
