- Played by Samantha Smith.
- Appears in: 1.01 Pilot, 1.09 Home, 2.20 What Is and What Should Never Be, 2.21 All Hell Breaks Loose: Part One
- Born December 5, 1954
- Dies November 2, 1983
- A draft script of the Pilot ages Mary in her late 20s at time of death.
- Kept extensive journals (see The Journal (diary entries))
- Mary's spirit haunted the house in which she died, appearing in the room that was Sam's nursery as a flaming figure, before she sacrifices herself in order to banish a powerful poltergeist in 2006. (1.09)
- In 2.20 What Is and What Should Never Be, it is Mary's birthday and the date is December 5th.