Is Alias Grace based off of a real story?
Yes. In this work Atwood retells the story of Grace Marks, a Canadian woman from the 19th century.
Yes, Atwood's novel is a fictionalization of the life of historical figure Grace Marks, convicted of the murder of her wealthy employer Thomas Kinnear in 1843. She was also involved in the murder of Kinnear's housekeeper, Nancy Montgomery; although trial for this crime was deemed unnecessary as Marks had already been sentenced to life in prison for Kinnear's death. Gaps in the case's historical record left room for interpretation by the author, combined with Marks' contradictory testimony.
Several plot elements are based on facts surrounding Marks' case, expanded upon by Atwood's fictionalization. Grace Marks was the first person to be deemed criminally insane in Canada due to her supposed amnesia surrounding the circumstances of the murder, as well as the trauma and abuse experienced in her youth. This gave room for Atwood to create the fictional character of Mary Whitney, the persona Marks assumes when confessing to the murders while under hypnosis. The ending of Alias Grace also reflects Atwood's artistic freedom. According to historical record, Grace Marks disappeared without a trace after her release from prison; in the novel, Atwood creates the plot line of Marks marrying Jamie, a young farm hand.
According to the Afterword of the McClelland & Stewart 1996 publication of Alias Grace: Atwood's opinion of her protagonist changed upon reading widely that Marks' lawyer fabricated parts of her third-person testimony of the murders. This fact motivated Atwood to create her own interpretation of Marks, in which her innocence is left up to the reader to decide. Smithsonian Magazine provides further fascinating detail on the true elements of Marks' case, allowing for a deeper perspective on where criminal history ends, and Atwood's narratives begin.