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. While the family eats dinner together, Mary complains about Zoey using the word 'penis', though Zoey could care less about her mother's opinion.
After locking down for Purge Night, Mary spends some alone time running on a treadmill.
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 pretty poorly, not killing a single purger.
As Mary walks through the house, a male Purger creepily watches and stalks her from afar.
Later, as the neighbors start killing the Purgers, Mary is overpowered by one of the female purgers, who decides to play with Mary. As she torments Mary by tickling her ribs, she taunts her ticklish victim as she squirms under her. The Female Purger cackles at Mary when she begs the tickling to not stop as she raises a machete, only to be shot in the head by Mrs. Halverson.
- Mary is ticklish, as shown when the Blonde Tickling Purger attacks her.