Joan was far brighter than Frank and, after graduating from school with 7 O-levels and an A-level, she married a chief accountant, Graham Garwood. Following their marriage, Joan and Graham moved to Colchester, and kept little contact with Joan's family in Walford. After Mo is diagnosed with Alzheimer's disease in 1990, she starts speaking of and referring to people (mostly Diane Butcher, her granddaughter) as Joan, which gives some light into their relationship: Mo believes that Joan sees herself in a higher standing than the rest of her family, which earns Mo's dislike and disapproval.
As her mental state deteriorates, Mo causes a fire in her flat after dropping a cigarette in her linen, which leads to Mark Fowler rescuing her, and Mo being hospitalised overnight. After this, Frank contacts Joan and she visits Walford to assess the situation, calling a truce with Frank; the siblings had not spoken since the funeral of Frank's first wife, June. Joan also meets Janine properly for the first time, and, when questioned by her, reveals that she has no children because of fertility issues. Throughout the day, Frank and Joan snipe at each other about their lack of contact, until Pat Evans intervenes. When Joan goes to leave, using her job at an Oxfam as an excuse, Mo suddenly becomes lucid and begs her to stay overnight. Later, Joan argues with Frank over her involvement in the Butcher family, accusing Frank of not knowing her at all, and also revealing that Mo never wanted a daughter and thus ignored Joan in favour of Frank. After the argument, the two read a letter Mo wrote to Frank before her condition deteriorated, in which Mo asks Frank to put her "out of her pain".
The next day, Frank and Joan have tea with Mo, who seems to have become more lucid and reminisces with them about their childhoods. However, Mo believes that Frank wants to kill her, having forgotten about the letter she wrote to him. In a heated confrontation, she smashes a teapot. Joan leaves Walford that evening, and agrees with Frank that they should keep in contact for Mo's sake, although her departure is marred by Mo's insistence on leaving with her - she is still afraid that Frank wants to kill her. Joan placates her by saying she will talk to Graham first.
Unable to cope with his mother's illness, Frank begins talking about fulfilling the wish in Mo's letter, worrying Joan. The next day, in more lucid moments, Mo phones for minicabs to take her to Colchester, calls which Frank and Dot Cotton have to cancel afterwards. Joan and Graham visit Walford that day and Joan suggests that she and Graham take Mo to stay with them temporarily, saying that the break may do Mo's memory some good and give both Frank and Pat a chance to relax. However, when Frank points out that Mo's memory will never get better, Joan accuses him of not wanting Mo to get better. She and Frank then argue, with Frank accusing Joan of only getting involved out of guilt, due to her estrangement from the family and ignoring Mo after the death of their father. Joan and Graham then take Mo to a restaurant, as Mo believes that Pat and Frank are trying to poison her and refuses to eat another meal served there. Afterwards, when Joan leaves, Mo begs her not to and hides from Frank when he comes into the room, bursting into tears, believing he will kill her. After a brief talk with Dot, it is decided that it is better for Mo to go to Colchester with Graham and Joan, and Dot explains to Frank that it will be better to let her go than to keep watching her decline. The three leave that night, and Mo lives with them until she dies on 31 December 1992.
Following Frank's disappearance in 1994, his family start up a search for him that continues into the next year. Over Christmas that year, Pat visits Joan to see if Frank is staying with her. Joan tells Pat she has not seen Frank for several years.