Walter Kendall Myers, 72, and his wife, Gwendolyn, 71, were caught in an undercover FBI sting operation, arrested in June and held without bail.
Two people familiar with the plea deal, speaking on condition of anonymity because they were not authorized to discuss the negotiations, said Walter Kendall Myers agreed to a life sentence partly to help his wife get less prison time in the hopes she would not die behind bars.
Walter Kendall Myers pleaded guilty to plotting to commit espionage and to wire fraud.
His wife pleaded guilty to the lesser charge of plotting to gather and transmit national defense information and agreed to serve six to 7 1/2 years in prison. Both agreed to cooperate fully with investigators.
U.S. District Judge Reggie Walton accepted their pleas Friday afternoon.
The couple's lawyer Bradford Berenson said they acted "not out of selfish motive or hope of personal gain but out of conscience and personal commitment."
"They always understood that they might some day be called to account for that conduct and always have been prepared to accept full responsibility for it," the lawyer said in a written statement.
Through court documents filed in connection with the plea, Walter Kendall Myers admitted he was known as "Agent 202," and that he and Gwendolyn began a conspiracy in 1979 to provide national security information to the government of Cuba. The couple married three years later.
U.S. authorities say the Myerses delivered government secrets to Cuban agents over the past 30 years using a shortwave radio, by swapping carts at a grocery store and in at least one face-to-face meeting with former Cuban leader Fidel Castro in Cuba.
Key evidence in the case came from an undercover sting involving an FBI operative who approached Kendall Myers on the street on the defendant's birthday, April 15. The operative gave Kendall Myers a cigar, said he knew his Cuban handler and asked that they meet later.
The ruse worked, and the Myerses met three times with the undercover operative at Washington hotels over the next two weeks. The FBI secretly videotaped the sessions, in which they say the couple made many incriminating statements about their time as spies.