Whitey Bulger, 82, was one of the FBI's most wanted men. He is charged with participating in 19 murders as head of Boston's notorious Winter Hill Gang.
Greig, 61, pleaded guilty to conspiracy to harbor a fugitive, conspiracy to commit identity fraud, and identity fraud.
Greig's lawyer said she doesn't regret her life because she still loves Bulger.
"This was not a romantic saga," U.S. Attorney Carmen Ortiz said. "She helped and protected and concealed a fugitive."
Prosecutors asked the judge to sentence Greig to 10 years in prison, citing the length of time she helped Bulger and the number of things she did for him, including taking him to medical appointments, posing as his wife so she could pick up his prescriptions and using false identities.
Greig and Bulger posed as married retirees from Chicago and had a stash of more than $800,000 in cash and 30 weapons in their apartment when they were captured last June in Santa Monica, Calif. Bulger and Greig were apprehended just days after the FBI began a new publicity campaign focusing on Greig.
The Associated Press contributed to this report.