Nevine Aly Elshiekh, 47, and her boyfriend, Shkumbin Sherifi, 22, pleaded guilty to one count of conspiracy to commit murder and will be sentenced February 4. The maximum sentence is 10 years in prison, plus probation and a $250,000 fine.
They both agreed to cooperate with prosecutors in exchange for the dismissal of other charges.
The two allegedly conspired with Sherifi's brother Hysen to behead a witness who was testifying against Hysen in a federal terrorism case.
FBI agents previously testified they faked photographs to make it look like the murder had been carried out.
Hysen Sherifi was sentenced to 45 years earlier this year in what prosecutors described as a conspiracy to attack the Marine base at Quantico, Va., and targets abroad.
Three other men in his Johnston County, North Carolina-based terror cell pled guilty to terrorism charges in 2011, and three more defendants were found guilty at trial. Another defendant still awaits trial, while an eighth suspect remains at large.
According to an affidavit, Hysen Sherifi confided in an informant that he wanted to kill three witnesses who testified against him and a prisoner he believed had stolen from him.
Shkumbin allegedly paid an FBI informant $4,250 in cash toward the first killing.
Hysen Sherifi's trial on the murder conspiracy charges was set to begin November 5.
Nevine Aly Elshiekh was allowed to remain free on bond. Shkumbin Sherifi has been in jail since his arrest.
Shkumbin Sherifi and Nevine Elskeikh will both be sentenced on Feb. 4, 2013.
ABC News and the Associated Press contributed to this report