Mary is the estranged daughter of Derek Harkinson. She has not seen her father since he walked out on her mother, Cynthia, to live with another man, Robert. When Derek's friend Pauline Fowler finds out that Derek had children, she contacts Mary without telling Derek. Mary initially agrees to meet with Derek, but shuts the door in his face when he arrives at her house. Derek eventually persuades Mary to see him, and they meet, along with Mary's brother Alex Harkinson and his son Danny Harkinson. Derek later leaves Walford to live with Mary after a string of arguments with Pauline.