Koral Karsan, 50, pleaded guilty to attempted grand larceny in the third degree, admitting he threatened to embarrass Ono unless she gave him "more than $3,000."
His attorneys said that since the illegal immigrant already has been in jail 60 days, he can be transferred from a city jail cell to federal immigration custody.
Assistant District Attorney Anne Schwartz told the judge that prosecutors could have won a conviction on the original charge, punishable by up to 15 years in prison. She said they accepted the plea to protect the privacy of Ono, her family and friends, and residents of the Dakota, the apartment building where Ono lives.
Eliot Mintz, a spokesman for Ono, said she told him she feels "vindicated because this man has admitted his guilt."
Karsan was charged initially with first-degree attempted grand larceny for allegedly threatening to release embarrassing recordings and photos of Lennon's 73-year-old widow unless she paid him $2 million.
As part of the plea deal, Karsan read a statement in court in which he admitted he gave Ono a letter on Dec. 8, 2006, telling her "if you want all of these recordings, e-mails, conversations and memories to vanish from the face of the earth and never hear from me again, all you have to do is send me an amount more than $3,000." Any defendant pleading guilty to third-degree attempted grand larceny must admit trying to steal at least that much.
Soon after Karsan's arrest in December, Schwartz said the defendant had told Ono he "had people on standby waiting to kill her" if she didn't pay him.
Outside court Friday, defense attorney Robert C. Gottlieb said Karsan took the plea deal because he would have been in jail months longer waiting for a trial. He said his client was an aggrieved employee "who was sexually harassed" and otherwise mistreated by Ono and was demanding compensation, not blackmailing her.
"What happened in that letter goes on between lawyers every day," Gottlieb said, calling the letter "inartfully" drafted. "His mistake was in not hiring a lawyer to be his mouthpiece."
Karsan now faces deportation to his native Turkey. His immigration lawyer, Jonathan E. Avirom, said the defendant will be handed over to immigration officials whenever they want to pick him up from Rikers Island.
On the night of Dec. 8, 1980, 26 years to the day before Karsan gave Ono his written demands, Lennon and Ono were returning to the Dakota where they lived, and where she still lives, when a gunman shot the ex-Beatle four times, killing him.