Joanna was a young widow who lived alone outside of Collinsport. She used to work for Randall Drew before she met Quentin. Quentin and Joanna used to meet secretly, and left notes to one another in an old oak tree on the Collinwood estate (1140). Joanna and Quentin shared a piece of piano music, entitled "Joanna", and this melody became closely associated with the couple's relationship. When Quentin realized that his wife, Samantha Collins, would never leave him, he broke it off with Joanna, devastating the girl and forcing her admittance into an asylum. Enraged and jealous, Samantha pushed Joanna Mills off of a cliff, seemingly killing her (1182).
Following the official declaration of her death, Joanna's trunks of personal belongings were sent to Collinwood and into the care of Samantha. Inside, letters written but never sent to Quentin, as well as the sheet music for the song "Joanna", gave Samantha the ability to make her husband believe that his mistress had come back from the dead.
In 1840, however, during Quentin's witchcraft trial, Joanna reappeared, testifying in court on Quentin's behalf and causing the previous testimony of Lamar Trask to be thrown out. Pretending to have no memory of her encounter with Samantha, Joanna began visiting her sister Daphne at Collinwood. Always polite, she enraged Samantha, who believed that Joanna had come to Collinsport to kill her (1181).Joanna offered Quentin the chance to be with her, but Quentin explained that he had fallen in love with Daphne (1183). Later, while searching for her missing sister in Collinwood's abandoned east wing, Joanna witnessed Parallel Time inside the parlour (1190).
Joanna assisted in Quentin's escape from the authorities (1189) and kept watch for the couple as they met in the east wing of Collinwood. Joanna and Samantha spoke in the drawing room, and Samantha fired a gun at her. Although she was sure that Joanna had been hit and somehow not been killed by the shot, Joanna claimed that the bullet had missed her (1192). Having accepted that Quentin and her sister were in love (1191), and that Samantha would be an obstacle to their happiness, Joanna later lured Samantha to Widows' Hill with a note forged in Gerard Stiles' handwriting. There she revealed that Samantha had been successful in killing her, and that her true face was that of a skull (1192). Samantha turned to escape, and fell from the cliff. Joanna was seen at Collinwood the following morning, but never showed her real form or admitted her actions the previous night to Daphne or Quentin (1193). It can be assumed that following Samantha's death, Joanna returned to her own grave and could finally rest in peace.