Detective Mackey was played by Christopher Plummer in the 1995 movie adaptation.
Detective Mackey was the detective responsible for the investigation of the Joe St. George case. He rightfully believed, that Dolores Claiborne had killed her husband, but couldn´t prove it. When Vera Donovan dies he opens an investigation into her death and ties it to the older case. The investigation is ended when Selena decides to protect her mother. He has no choice, but to close the investigation and let Dolores go.