The Bucks County District Attorney's Office says Mary Jane Fonder died Monday at the State Correctional Institution at Muncy.
Fonder, 75, committed one of the "most notorious murders in Bucks County history" and was suspected in the 1993 disappearance of her father.
Corrections officials say she died of cardiac arrest in the prison infirmary.
Fonder had been in prison since December 2008 after she was sentenced to life in prison for the murder of receptionist Rhonda Smith, 42, in the office of a Springfield Township church.
Officials say Fonder was infatuated with her pastor, but when he showed no interest in her she thought of Smith as a rival for his affections. Thus, Fonder fatally shot her.
The DA's office says the murder went unsolved for over two months.
In addition, Fonder is also suspected of killing her 83-year-old father Edward Fonder III. Police say he "vanished" in July 1993 from the house he shared with his only daughter. The DA's office says the search for Fonder's father will continue despite her death.