Sonia Jackson arrived in Albert Square in 1993 with the rest of her family. She was the third of Carol Jackson's four children and was fathered by Terry Cant, a fling by her mother and someone she never knew. During her early childhood, Sonia became good friends with Clare Tyler until Clare fell in with the wrong group and began bullying Sonia. They managed to patch things up before Clare left in 1998. Sonia briefly left the square in 1997 after her younger brother, Billie's kidnapping but returned permanently to live with her older sister, Bianca in 1998.
Following a brief romance with an Italian exchange student, Sonia slept with Martin Fowler. After putting this behind her, she began dating Jamie Mitchell and was stunned to give birth to Martin's daughter in 2000; she hadn't realised she was pregnant, putting her weight gain down to overeating. Baby Chloe (later renamed Rebecca) was put up for adoption despite protests from Martin's mother, Pauline. Sonia later developed an obsession over her daughter, abducting her and locking herself in her room. Despite pleas from Rebecca's adoptive parents, it was Dot Cotton who managed to persuade Sonia to come out and give back the baby.
Sonia and Jamie got engaged but after Jamie's fling with Zoe Slater and their constant arguing, the engagement was called off. Sonia began dating Gus Smith who planned to propose but was left devastated after Sonia left him to go back to Jamie after he is assaulted by his guardian, Phil Mitchell. Sonia nursed him back to health and they planned to get married. Their happiness is short-lived as Martin Fowler accidentally hits Jamie with his car in December 2002. Sonia found it hard to let Jamie and her anger towards Martin go but eventually forgave him. They eventually became close, got married and eloped together.
Sonia began nurse training and lived with Martin and his mother, Pauline but found her interference hard to cope with. Months into the marriage, Martin became unwittingly involved with stalker, Sarah Cairns, who was out to ruin his marriage. This culminated in Sarah holding Martin and Sonia hostage and stabbing Martin before Sonia knocked her unconcious. Sonia and Martin faced further upset when they discover Rebecca's adoptive parents were killed in a car crash. They visited Rebecca's guardian, Margaret, whilst protecting their true identities. When Margaret found out who they were, she ordered them to leave the house. Pauline interefered and, alongwith Martin, visited Rebecca. Sonia was furious when she discovered this, the animosity in the Fowler household put a strain on their relationship. Sonia began spending time with her nursing colleague, Naomi Julien which led to a lesbian affair behind Martin's back. Martin was heartbroken over the ending of his marriage and this was further complicated when, under the request of Margaret, Rebecca spent more time with the Fowlers. Margaret decided against Sonia being allowed to see her after learning of her lesbian fling. When Margaret died and custody of Rebecca was given to Martin; Pauline began stopping Sonia from having anything to do with her biological daughter. As a result, Sonia became distraught and preoccupied with Rebecca leading Naomi to end their relationship.
Sonia became depressed, neglected her work and began drinking. Gus Smith helped straighten her life out and they began a second relationship, which Sonia ended due to her lingering feelings for Martin. In October 2006, Martin and Sonia's divorce papers came through however both felt remorseful and began their relationship again. Pauline discovered this and concocted a story where she was dying from a brain tumour so Martin would dump Sonia and look after her. Martin ended his relationship with Sonia, just as Pauline intended, but when the truth finally came out, Martin moved out. Pauline responded by threatening to cut her son out of her life.
Sonia attempted to build bridges with Pauline on Christmas Day 2006 which resulted in an argument where Sonia slapped Pauline. Pauline was found dead in the square later that night. Sonia feared that her slap was the cause of Pauline's death and after discovering Rebecca had witnessed the assault, Sonia tried to silence her daughter. Rebecca later told Dot at Pauline's funeral. When Martin found out, he was appalled and stopped Sonia's access to her daughter. Fearing imprisonment, Sonia went on the run and abducted Rebecca. Martin frantically searched for his daughter but Sonia returned her days later and was arrested on a murder charge. That night, Dot uncovered the real identity of Pauline's killer; Joe Macer who had hit her over the head during an argument. Sonia was released and decided to leave Walford in 2007. After making peace with Martin, Sonia took a taxi and was about to leave before seeing Martin and Rebecca obstructing the road. Martin had decided he wanted to leave with her and the Fowlers reunited. After spending time touring the USA, they settled in Manchester.
In January 2010, Bianca contacted Sonia to invite her to her wedding to Ricky Butcher. Sonia texted Bianca saying that she and Martin would attend. Sonia arrived in Walford alone in Feburary, saying Martin and Rebecca were doing fine but later rejected a phone call from him and got very drunk. The following day, Sonia walked in on Dot slapping her granddaughter, Dotty Cotton, across the legs. This led to her arrest but she was released without charge. After a week, Bianca noticed that Sonia continously avoided talking about Martin and Rebecca. Sonia visited Jamie's grave and returned to the Queen Victoria pub before laying into Phil Mitchell, verbally about the treatment of Jamie when he was alive. Phil calmed her down, telling her that he considers Jamie one of his kids and told her to go upstairs to sober up. Sonia misunderstood and got into Phil's bed, naked. As soon as Phil walked in on Sonia, Shirley Carter (Phil's girlfriend) walked in, got the wrong idea and slapped Phil across the face. Sonia later apologised to Phil after a telling off from Shirley. She later confided in Carol about her marriage problems with Martin and was convinced to give it another go for Rebecca's sake. After the wedding of Bianca + Ricky, Sonia was inspired to start afresh with Martin and left Walford once again. In October 2010, Sonia revealed that she couldn't attend Billie's funeral because she was visiting her other brother, Robbie in Mumbai but she sent flowers. Sonia returned to Walford in 2011 to reveal where Bianca was as she had gone missing. She told Carol that Bianca was staying in a B&B in Dagenham and explained that she was too upset to attend Billie's funeral, not because she was in Mumbai. She also revealed that she and Martin had split up because he was having an affair and that she and Rebecca were living in London. She then took Carol to see Bianca.
Natalie Cassidy portrayed Sonia throughout her time on screen.