Mary Pickford (also known by the nickname Scary Mary) is the alias adopted by Edna Strickland after she took a malfunctioning DeLorean time machine back to 1876. She kept the name through 1931A, where she was visited by Marty McFly and Emmett Brown, who went back to 1876 after meeting her in order to restore history.
For a full biography, see the article on Edna Strickland.
After being ousted as the Speakeasy Arsonist by Marty McFly in 1931, Edna attempted to escape from officer Danny Parker in a temporal clone of the DeLorean, unaware that it could travel through time. Escaping in the car caused her to be sent back to 1876, where she adopted the name Mary Pickford. As Pickford, Edna found the nineteenth century Hill Valley to be much more peaceful than her home time of 1931. Edna's peace was soon disrupted when she caught wind of Beauregard Tannen's plan to build the Palace Saloon. She attempted to turn him into Marshall James Strickland, who did not take action, as alcohol was not illegal in the United States at the time and would not be for close to half a century. She decided to repeat her actions of the future, and planned to burn down the saloon on July 17. She underestimated the strength of the fire, as it ended up burning down the rest of Hill Valley. This resulted in the branch into 1931A, where Edna, having repressed her memories of the events, lived with dementia in a small house in what would have been Hill Valley.
In 1931A, Marty and Doc visit the delusional "Mary" (dubbed "Scary Mary" by the locals) to try and pinpoint the exact moment in time where history split in order to go back and repair it. Due to her 55 years of isolation and resulting dementia, Edna was unable to give them any information on the events of the town-leveling fire. To attempt to jog her memory, Marty used Doc's presence and other random objects scattered around her yard to see if she could fill in the blanks and recall the entire event. Marty's tactic succeeded, and Edna brought him and Doc a newspaper with the exact date of the incident. Thinking they would report her for her actions over fifty years ago, Edna pointed a gun at the two, only to be stopped from shooting them by local passerby William McFly.
Marty and Doc then went back to 1876 to stop the events leading up to the fire. By the time they arrived, Edna was already setting down the fuel to begin her plan. She was confronted by Beauregard Tannen, who threatened to shoot her for trespassing. This resulted in a confrontation between the two of them (Edna holding a torch over the fuel and Tannen pointing a gun at her), and Doc Brown (known by Edna only as "Carl Sagan"), who attempted to point out to Tannen that shooting Edna would only result in her posthumously dropping the torch, still starting the fire. Tannen apathetically continued to hold the gun pointed in Edna's direction. The confrontation was "quelled" by Marty dropping sandbags on both Edna and Tannen from the rafters of the unfinished saloon, putting out the torch and prompting Edna to escape. In a chase between Marty and Doc in the DeLorean and Edna in the clone she stole from them, The two were able to drag her back to a 1931 untouched by the saloon fire, where she was arrested by Danny Parker.