Lisa arrives in Albert Square as a trainee market inspector, but clashes with her boss Michael Rose because of her poor time-keeping. However, she starts to fall for him, and the pair strike up an affair. Lisa soon begins to feel guilty, and demands that Michael chooses between her and his wife Susan. Michael initially chooses Lisa, but does not tell Susan, and Lisa threatens to tell her. To escape the breakdown of his marriage, he flees Walford with Susan in February 1999 , leaving heartbroken Lisa behind.
She later makes friends with Mel Healy and moves in with Mark Fowler. Although Mark has feelings for her, Lisa does not notice, having a fling with Gianni di Marco and a relationship with Phil Mitchell, resulting in Lisa's pregnancy. Phil asks her to have an abortion but she refuses so he asks her to move in after coming round to the idea but she miscarries and blames Phil. Lisa loses her job when Ian Beale reports her for being absent, so she becomes dependent on Phil. He is happy with the situation but she loses her confidence and becomes jealous of Phil's former wife, Kathy Mitchell and son Ben, hiding letters and a video from them. This irritates Phil and he has sex with Mel when she is upset, following a fight with Lisa on Christmas Day. Regretting this, Mel agrees not to tell Lisa but does when Lisa says she intends to get pregnant again. Devastated, Lisa ends her relationship with Phil and moves in with Mark but she is already pregnant. She keeps this secret as Phil keeps belittling her, and on Mel and Steve Owen's wedding night, Phil says that he never loved her and suggests Mel give her tips in the bedroom. In revenge, Lisa steals a gun from the E20 nightclub and shoots Phil. He survives and confronts her after leaving hospital but realises that he drove her to it and frames his enemy Dan Sullivan instead. Wanting to keep Phil away from her child, Lisa and Mark marry, claiming that he is the baby's father. Everyone believes it and their romance becomes real. Lisa gives birth to baby Louise, and Lisa tells Sharon Watts, that Phil is Louise's father. Sharon, however, tells Phil that Louise is his daughter and he forces them to accept he will be part of Louise's life. Despite initial animosity, when Lisa sees Phil with Louise, old feelings resurface and they eventually reconcile. However, Phil wants Louise, not Lisa, and when she realises, she and Louise emigrate to Portugal. Phil finds her and brings Louise home after blackmailing Lisa about shooting him and making her think she is unstable and an unfit mother. Phil leaves her standing on the edge of a cliff, and she is presumed dead. On Phil's wedding day to Kate Morton, Lisa returns to reclaim her daughter. He is difficult, leading her to plan to shoot him again but Den Watts persuades her to let him do it. He and his son, Dennis Rickman, frame Phil for armed robbery and Lisa reclaims custody when he is imprisoned. She leaves the Square with Louise.
Several years later, Jack Branning, a former policeman, tells Phil that Lisa and Louise are living in South East London. After seeing Louise call another man "daddy", Phil decides that she is better off with Lisa. Two years later, Louise arrives alone in Walford, saying that Lisa has gone on holiday but not returned. Louise is taken into care but when Phil discovers this, he has a DNA test to prove Louise is his daughter and gains custody of her. Later that year, Phil is visited by a social worker, Derek Evans, who says that Lisa has made an application to see Louise. Phil does not want Lisa to have contact with Louise, but Phil's mother Peggy Mitchell takes her to see Lisa on Louise's request. Peggy slaps Lisa for abandoning her daughter, and Lisa reveals she had a breakdown and thought Louise might be better off without her, but the neighbour she left Louise with promised to look after her. Lisa worries that Phil might hurt Louise physically after Phil has hit Peggy. Peggy is unable to assure her that this will not happen. Peggy allows Louise to stay with Lisa permanently as long as Phil can visit her. Phil steals the social worker's bag to get Lisa's address, and drives there to confront her but finds the house abandoned and empty.
Seven years later, Louise is burnt by bullies Madison Drake and Alexandra D'Costa, and Lisa visits her in hospital. Lisa and Sharon argue but Sharon allows Lisa to stay. She tells Sharon that Louise wanted to live with Phil when she got a new boyfriend, which she admits was a relief. Lisa tells Phil she has had help with her life and has changed, but he ejects her from the hospital, telling her to stay away from Louise. She then stands up to Phil and he allows her to visit Louise. Lisa then starts talking strangely. She sees her therapist and lies that she has been doing fun things with Louise. Lisa takes keys to Phil's house and lets herself in, talking to and smashing photos, but cuts herself on glass. A doctor allows Lisa to take Louise outside of the hospital, but Lisa tells Louise she has been discharged, and takes her to a train station. Phil and Sharon inform the police that Louise is missing, and they trace Lisa through a cash withdrawal. Phil then discovers that Lisa has a mental health team. Lisa takes Louise to a hotel and prevents her from using the phone by cutting the cord, despite her being in agony. Louise realises that Lisa is not taking her medication. When Lisa allows Louise to go, she chooses to stay and comforts Lisa when she is distressed. Phil tracks them down but Lisa hits him over the head with a phone while Louise passes out. Sharon realises that Lisa's problems are all Phil's fault and Lisa tells Louise they will go to the hospital together. Later, Louise blames herself for Lisa's condition, telling Phil she left her knowing she was ill.
In August 2019 , a heavily pregnant Louise and her fiancé Keanu Taylor flee Walford after Phil's son Ben, frames Keanu for attacking Phil, when the real culprit was Stacey Fowler. Ben urges Mel into giving him Lisa's address and Mel lies to Ben about Lisa's whereabouts, although Mel eventually reveals her location, Lisa is not found. Lisa returns in September 2019 to seek Mel's help to get Keanu out of the country. Lisa suggests that she, Louise, Mel, Keanu and Mel's son Hunter Owen flee together, but Hunter escapes and holds The Queen Vic hostage. Hunter takes Louise, threatening to shoot her if the police do not give into his demands, but later he is later shot and killed by a marksman. Phil asks Louise and Keanu to move back in with him, which upsets Lisa, as she had planned for her and Louise to live together. A concerned Lisa, recognising the signs, tries to support Louise's best friend Bex Fowler, advising her that she doesn't have to go to Oxford University if she doesn't want to, but Bex ignores her, pretending she's fine; Bex later overdoes.
Lisa and Mel fall out over Sharon and also over Mel's increasing obsession with Louise's pregnancy. However, when Mel is killed Lisa is devastated and, genuinely believing Sharon killed Mel, she exposes to Phil the truth she learnt from a vengeful Mel that Phil is not Sharon's baby's father. She later begins a feud with Sharon and on the day of Mel's funeral, she bans the Mitchells from attending and reveals to the whole pub that Sharon's baby is not Phil's. After the square thinks Lisa is mentally ill again, she checks herself into a unit for a break. However, Sharon reveals to Lisa that Keanu is the father, as Louise gives birth. Upon hearing this, Lisa leaves the unit, with the intention to tell Phil. Once Lisa sees them as a family, she decides not to tell Phil and Louise, but makes sure that Keanu never hurts Louise, stating that if he does, she will reveal everything. The birth of their granddaughter, Peggy Taylor, brings Lisa and Phil together and they make amends. Phil, however, becomes suspicious and starts to believe Lisa. Mistakenly thinking Phil and Ben arranged Keanu's "murder", Lisa, Louise, Peggy and Phil flee to Portugal but Ben remains in Walford.