Mary is the middle child of The Sanderson Sisters, a trio of witches who lived in colonial Salem. Together, the three would use magic to lure away children and steal their youth, in order to gain youth and immortality. Of the three, Mary has the ability to smell and track children. In collaboration with her sisters, Mary can curse an entire crowd via singing and always tries to get Winifred's attention.
Mary is known to trying to get Winifred's attention, and tends to be the calming influence to her. She is not as ditzy as Sarah, and in fact tends to be the most observant of the three. She is sometimes seen barking like a dog, though this usually happens when she tries to be intimidating.
Mary has black or dark brown hair, and wears clothing in shades of red. She is slightly overweight when compared to her sisters.
At the beginning of the film, Mary is seen assisting Winifred in the creation of a potion that will allow them to steal the youth from children. Meanwhile, Sarah lures young Emily Binx from Salem to be the victim, unaware that Emily's brother Thackery is following them. The sisters are able to successfully steal Emily's youth, effectively killing her and causing them to become younger. After discovering Thackery, Mary assists in turning him into an immortal black cat so that he may live with the guilt of being unable to save his sister. Just then, Salem's villagers arrive, and become aware of the witches' actions after an outburst from Sarah. Mary is hung alongside her sisters, though Winifred casts a curse that will one day allow them to return via a virgin lighting a Black Flame Candle.
300 years later, on Halloween night, the sisters are brought back to life by Max Dennison, a teenage boy who did not believe in the curse. After Max, his sister Dani, and his crush, Allison, steal Winifred's spellbook, the sisters are forced to follow them; they must suck the lives of children in order to live past dawn.
Mary and the others track the kids to a cemetery, where they are forced to remain on their broomsticks. Thus, Winifred raises Billy Butcherson from the dead and commands him to capture them, though he is unsuccessful. Later, Mary and the others get a bus driver to take them to find children to be victims. Mary initially believes that she has lost her powers when she smells children though doesn't see any. At this point, they meet a man dressed as the devil, and believe him to be their master. However, after being run out by the man's wife, they realize that Halloween is a time when people wear costumes. During this time, their broomsticks are stolen, forcing them to continue on foot.
The sisters then follow the children to the party where Max's parents are and after being exposed to the room by Max, Winifred takes charge and ridicules him before singing "I Put a Spell on You" therefore enchanting all the adults causing them to dance until they die. After this the children lead the sisters to the school after Allison gets an idea from a restaurant and after unwittingly walking into a kiln they are locked in and the children burn them alive.
However, they did not die due to the curse protecting them from death until sunrise and they end up capturing and locking up Jay and Ernie, two bullies that had made fun of Max after they call them ugly. Winifred, desperate for her eternal youth decides to try and make the potion from memory. Sarah, surprisingly, shouts out the correct ingredient but is hushed by Mary who thinks Winifred has all the right answers. At Max's house, though, he and Allison open the book therefore allowing it to send out a signal to the witches who fly to the house, with Mary using a vacuum cleaner to replace her stolen broomsticks. They take the book and kidnap Dani and Thackery. Mary accompanies Winifred to the house, and assists in making the potion.
After brewing the potion again they try to get Dani to drink it but are stopped by Max who tricks them into thinking the sun has risen by using the headlights of his parents' car. However they realize they have been tricked and follow the children back to the cemetery and attempt to get Dani to drink the potion again though Winifred drops it whilst Sarah tries to help Mary escape the children. Max catches it and threatens to smash it though instead drinks it after Winifred says she'll kill Dani if he smashes it. After a struggle with him, they both fall to the ground and Winifred starts to take his life force but the sun comes up. Winifred turns to stone and explodes whilst Sarah and Mary implode in purple and red light.
At the end of the movie, however, the book is seen and it blinks, therefore hinting the sisters may return once again.