When Arthur Miller first introduces Mercy Lewis near the beginning of act 1, the stage direction says, "Enter Mercy Lewis, the Putnams' servant, a fat, sly, merciless girl of eighteen." Early in the play, Mercy cares for Ruth Putnam, the other child who appears to have taken ill after the girls were caught in the forest dancing around a fire with Tituba. Mercy is also, evidently, a very good friend of Abigail Williams, the Reverend Samuel Parris's niece. When Abigail threatens Mary Warren, Mercy joins in, menacing Mary when Mary indicates that she wants to confess to what they did in the forest.

Later, in act 3, when Abigail accuses Mary of witchcraft in the courtroom, Mercy joins in again. She speaks with "chattering teeth" and asks Mary, "Mary, do you send this shadow on me?" By accusing Mary of being a witch, Abigail and Mercy discredit Mary's testimony against them. Finally, in act 4, when Reverend Parris tells Deputy Governor Danforth that Abigail has stolen his life savings and fled town, he reveals that she and Mercy Lewis were in on the plot together and that both have disappeared.