At first their relationship began antagonistic towards each other, due to Mercy being under Mary's control. Mercy was frequently used to falsely accuse others for witchcraft, but her father, Reverend Lewis later performs a Catholic exorcism in Survivors freeing her from the familiar within and Mary's control. She soon posed a threat to Mary as she could have accused her of witchcraft but instead of killing her, she chose to turn her into a witch in The Red Rose and the Briar so that she couldn't reveal what Mary was.
Mary became a strong female influence on Mercy, taking her under her wing and teaching her the craft. But with the arrival of Increase Mather who becomes suspicious of Mary. Mercy decides to accuses Mary's devoted servant Tituba for witchcraft to protect her new mistress leading to Tituba's arrest in Children Be Afraid and for a time incites Mary's wrath.
Not much is known of their earlier history but it is presumed that Mary knew or knew of Mercy as she is the daughter of Salem's reverend Lewis.
Throughout The Salem Series
In The Vow Mary stands in Mercy's bedroom in her hag form but can only been seen by Mercy. She proceeds to attack her once Cotton Mather and John Alden leave the room. The following day Mary controls Mercy at the market as she is paraded through the streets by Cotton. She has her incriminate Giles Corey of being a witch.
In The Stone Child following the arrest of Bridget Bishop Mary is forced to control Mercy during Bridget's trial to incriminate her further and has Mercy react to her presence by vomiting blood on her.
In Survivors Mary tries to control Mercy at the market when she is with her father but comes to realise that the familiar has been cut from her leaving her no longer in control. That night she visits Mercy in her hag form and forces her to swallow another familiar but later Mercy appears as an apparition in Mary home, revealing she knows who is her tormentor and what she is.
In Lies with the realisation that she cannot control Mercy. Mary visits her and asks her what she wants, she tells her she wants all the pain and torment she has caused her to stop or she will tell everyone who and what she is but Mary warns her that she will join those in the crag if she tries. Later that day Mary witnesses as Mercy along with her friends falsely accuse the father of Emily Hopkins at the market of being a witch. That night Mary spies on Mercy with her friends in the woods where Mercy reveals she wants to be just like her.
In The Red Rose and the Briar following the revelation that Mercy wanted to be just like her. Mary takes her back to her home with the intention on finally killing her but she is unable to do the deed. Instead she takes her to the woods where she turns her into a witch.