Mary is the wife of James Sandin and the mother of Charlie and Zoey Sandin.
She is first seen welcoming her husband home as she is preparing dinner, while her son Charlie startles her with a bizarre remote control toy. At dinner, she complains about her daughter using the word 'penis' at dinner.
When Purge Night commences, she spends time running on a treadmill before changing into a comfortable sweater.
At one moment during the film's events, she stands against the purgers that break into the house, armed with two handguns, but she does very poorly, not killing a single purger. She is overpowered by one of the female purgers, who decides to play with Mary by torturing her, though this 'torture' is comprised of tickling her, much to her embarrassment.
This continues for a while, with Mary laughing and begging for mercy, though her tormentor continues to tickle her.