Mary is Winifred's right-hand witch, always trying her best to appease Winnie and calm her rages. In her quest to stay on Winnie's good side, Mary compliments her whenever she can and has a habit of repeating whatever she says. Although perhaps not as cunning or as evil as her eldest sister, she is more competent, clever, and observant than Sarah.
Mary is the most caring of the Sandersons, but only to her sisters. It is Mary that suggests forming a calming circle to ease their stress, and it is Mary who takes care of Winnie when she loses hope. However, when it comes to children, Mary delights in tracking them down with her powerful sense of smell and suggest various ways of cooking them, from barbecue to "shish-ka-baby".
Mary has dark hair twisted in a distinctive style similar to a witch's hat, a plump physique, and dark brown eyes. She wears a red vest that has several metal rings attached to the bottom over an orange shirt, a plaid skirt underneath a reddish-purple apron, and a red matching cloak.
Mary has beauty mark on the right side of her nose and a habit of biting the left side of her lower lip, giving her face a lopsided look.
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 cottage, 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 transforms into stone and explodes while her two sisters implode in purple and orange light.
At the end of the movie, however, the book is seen and it blinks, therefore hinting the sisters may return once again.
She along with her Sisters began appearing in a Halloween-Themed Castle Show during Mickey's Not-So-Scary Halloween Party.
She along with Winnifred and Sarah try to brew a special Halloween Potion and receive assistance from various Disney Villains. After successfully completing the potion she joins Sarah in singing and dancing backup while Winnifred sings "I Put a Spell on You".