Mary Warren is one of the girls included in the circle of Abigail Williams. They are supposed to be key witnesses in the witchcraft trial. After a day of attending the trial, Proctor insists that she has a job with them and she should not be neglecting her duties to go to the trial. Mary defies Proctor and says she is an official of the court and, therefore, she must attend the proceedings. She then mentions that Elizabeth Proctor is a possible suspect in the investigation of witchcraft. Proctor convinces Mary that she must expose Abigail to the court but Mary is intimidated by Abigail when she goes to expose her. She joins Abigail's group once again and her behavior helps lead to John Proctor's arrest.
Mary Warren accuses various members of the community of being involved in witchcraft. Alongside Abigail and her posse of accusers, Mary says that Sarah Good is guilty of witchcraft after Sarah supposedly attempts to choke her by casting spirits during the proceedings. When Mary explains to John Proctor that she and the girls are doing "God's work," he threatens to whip her. Mary then mentions that she stood up for Elizabeth in court after Abigail accused her of being a witch. Essentially, Mary is Abigail's "puppet" who follows along and confirms everything she does and says. Even at the end of the play, Mary joins Abigail's side by accusing John Proctor of conspiring with the Devil.
Proctor does not want Mary going back to town for several reasons. He believes that it is Mary's duty to stay at his home and work because he is paying her nine pounds a year to do so until Elizabeth recovers. Proctor also believes that the witch proceedings are ridiculous. He does not believe in witches and is shocked to find out that thirty-nine people have been accused.