Former "Today" show co-anchor Meredith Vieira is to serve as host of the program.
"A recurrence of Harper's 2009 lung cancer, the disease spread into the membrane surrounding her brain and she was diagnosed with leptomeningeal carcinomatosis on March 6, 2013," a synopsis said.
"Since then, she has allowed camera crews unprecedented access into her life to help tell her story -- every step of the way. From the revelation that she had an incurable disease, to doctors' visits, second and third opinions, surgeries and reunions with family and friends, the documentary, airing on NBC in prime time, takes an intimate look inside Harper's reality."
The documentary's air date has yet to be announced.
Harper, 73, is best known for her work on the sitcoms "The Mary Tyler Moore Show," "Rhoda" and "Valerie."