In an interview with PEOPLE, Harper, 73, said she received the news Jan. 15. She had battled lung cancer in 2009.
Tests revealed she has leptomeningeal carcinomatosis, a rare condition that occurs when cancer cells spread into the fluid-filled membrane surrounding the brain. Her doctors say she has as little as three months left to live.
FOX News reports they spoke with Harper on Jan. 9, one week before her diagnosis. They said she was cheerful and upbeat, talking about her book, "I, Rhoda."
In the memoir, the New Jersey native writes candidly about her beginnings as a chorus girl on Broadway, eventually landing in Hollywood and the role that changed her life - Rhoda Morgenstern.
Maintaining a positive outlook on life, and resolved to face her last days with courage and humor, Harper said, "I don't think of dying ... I think of being here now."