Mary Todd Lincoln was the First Lady of the United States and wife of President Abraham Lincoln.
In the Timeless Universe
In the original timeline, Mary was sitting next to Abraham as assassin John Wilkes Booth sneaked behind the president and killed him with a shot to the back of the head at Ford's Theatre during a production of "Our American Cousin". Mary was laughing at the play, but as soon as the shot was fired she let out a shriek, and then started sobbing.
In the altered timeline, Abraham and Mary arrive to the play, late as always, according to their son Robert Todd Lincoln. When Mary arrives with Abraham, the national anthem is played throughout the theater, the play pauses, and everyone turns to look at the president enter. Robert introduces Lucy Preston, under the alias Juliet Shakesman, to his parents, Mary and Abraham. Mary sits next to Abraham. Due to Booth's refusal to cooperate with Garcia Flynn's plan, Flynn is the one who shoots Abraham instead. Lucy shouts at Abraham before the shot to try to warn him, but it is ineffective, but she manages to save Flynn's other target, General Ulysses S. Grant.
Mary Todd Lincoln at Wikipedia