Jase Dyer tries to flee Walford when he double-crosses a football firm but Dawn stops their departure by setting a date for her and Jase's wedding. Jase and Jay live safely in Walford for the next year. In January 2008, Jase gets involved with a local gang lead by Tegs. Tegs wants Jay to throw a brick through a window to finally be accepted into the gang but Jay refuses, his good side shows up and says she is a neighbour. Tegs stabs Jay and is later arrested. Jay goes on the straight and narrow.
Jay's father Jase is found by the football firm that are after him. The firm, led by Terry Bates, holds Jay hostage to get revenge on Jase. Jase is able to save Jay with Billy Mitchell's help but is beaten to death and Billy, too frightened to intervene, leaves Jase to die. Jay accuses Dawn Swann of causing his father's death and goes into care to get away from her.
Jay is later fostered by Billy, who is wracked with guilt for not saving Jase from the gang. Jay's behaviour worsens when he plans an arson attack on Dawn, but Billy is able to stop him and Jay eventually makes amends with Dawn. Jay and Billy move into a flat with Janine Butcher, who tries to get rid of Jay by stealing and blaming it on him. Feeling unwanted, Jay befriends Marissa Moore, who works at a brothel, and they plan to run away together. When Marissa backs out, Jay runs away alone and sleeps rough, until Billy convinces him to move back in. Terry's trial for Jase's murder approaches and Jay is furious to learn that, rather than attempting to help Jase, Billy hid to save himself. Terry's wife, Viv Bates, threatens to hurt Jay if Billy testifies. Jay is taken hostage by Terry's firm as a security measure, and Billy risks his own life to save him. Billy and Jay are both rescued by the police and Billy testifies, which results in Terry receiving life imprisonment for murder, partially redeeming himself in the eyes of Jay. Jay then becomes close to Phil Mitchell and he decies to adopt Jay and his name becomes Jay Mitchell.
Jay's girlfriend Abi Branning returns from her trip and learns from her parents Tanya and Max Branning about Jay's involvement in Heather's murder, and she rejects him. The next day she tells him he should have done more to stop Ben. He resumes his friendship with Lola Pearce and helps her steal cleaning supplies to make money, and they end up sharing a kiss in front of Abi. Abi insists she is happy for them both, however, Lola says she was just winding Abi up, and the three friends reunite. Jay gains employment from Max but is sacked when Phil refuses to work with Jay, when he has to collect cars from Phil. Abi consoles Jay, and after she is attacked by Alexa Smith, Jay comforts her and they declare their love for each other. Jay is later pleased when Lola asks him to be her daughter Lexi's godfather, he has doubts whether he will be good enough but Abi's grandmother and Lexi's godmother Cora Cross assures him he will be. In court, Jay is sentenced to community service for his part in Heather's death.
Jay, Abi, Lola, Dexter Hartman, Peter Beale and Cindy Beale Williams go on a caravan holiday, where Abi discovers her period is late and tells Jay that she could be pregnant. Jay and Abi argue because she wants an abortion if she is pregnant, while Jay would rather be a father. Abi is not pregnant but Jay thinks thinks their relationship is over, so kisses barmaid Kitty, witnessed by Dexter, who is Abi's cousin. Jay and Abi reunite but he feels guilty, and when Kitty keeps calling him, he tells her to stop, but is overheard by Lola. Jay promises Lola that he only loves Abi, and Lola agrees to say nothing to Abi. Dexter reveals Jay's infidelity to Abi in an argument. Abi is devastated, but ultimately forgives Jay.
Peter's twin sister Lucy Beale is murdered by an unknown assailant, and Jay discovers that Max is having a fling with a detective involved in the case, Emma Summerhayes. He agrees to keep quiet for the investigation's sake. Emma and her colleagues organise an appeal to raise public awareness into the murder inquiry, and display CCTV footage of a man in a beanie hat on the same bus as Lucy on the night she died. Jay is skittish on seeing the footage and later burns the hat. He then makes arrangements to move away with Abi when she gets accepted into Bolton University. On hearing that the police have a new lead on the man on the bus, Jay makes plans to leave for Bolton with Abi immediately after saying goodbye to Phil at a barbecue run by the Queen Vic, but during the proceedings Summerhayes and her superior, DI Samantha Keeble arrive and question Jay (as seen in scenes available online) over a robbery at a shop near to where Lucy was killed, and it is revealed he was with another man at the time. He denies any involvement and is released without charge. He later confides in Shirley Carter that he was with Ben Mitchell on the day Lucy was killed, but he has only seen Ben twice since he was released from prison. Shirley forces him to tell Phil, and his initial anger leads Jay to move in with Abi for a while, and he plans to move to Bolton before Phil marries Sharon Rickman (Letitia Dean). Jay later reconciles with Phil and reunites him with Ben. It is revealed that, under Ben's instruction, Jay buried Lucy's phone and purse in the allotment. On the day of Sharon and Phil's wedding, Jay and Abi split when Abi realises Jay is harbouring feelings for Lola. When Phil is accidentally shot by Shirley, Jay and Ben are instructed by Ronnie Mitchell (Samantha Womack) to throw the gun in the canal. Ben phones the police in a last ditch effort for more time with Lola, Jay is arrested, and Lola and Lexi move to Newcastle alone. Jay receives a suspended prison sentence and a three-month curfew. Although he intends to join her when the curfew is finished, Lola later calls Jay to end their relationship. Three months later, Jay meets Linzi Bragg and is attracted to her. They start dating but agree to take the relationship slowly. Jay takes Linzi to the car lot for some quiet time, and leaves her there while he goes to buy condoms, but when he returns, Phil has demolished the car lot and Jay fears that Linzi has been killed until he sees that she was not in it at the time and Phil had in fact nearly killed his own daughter Louise Mitchell.
Jay is horrified to discover that Ben and Abi have begun a relationship and Abi later moves in with the Mitchells. Jay develops feelings for Lola again, who rejects Jay's offer to start a relationship. When Max tricks Ben into signing The Arches over to him, Jay discovers that most of the cars he has been fixing are stolen. Phil lets Max believe that the stolen cars have been located by the police and Max signs the business back to Phil. When Ben is arrested on suspicion of killing Lucy, Jay tells Sharon, Lola and Billy that Ben mugged Lucy. Jay and Lola begin a romantic relationship. Jay realises he could be implicated in Lucy's murder, as her phone and wallet have been handed to the police. He leaves Walford, taking Lola and Lexi. They return to Walford the next day to collect their belongings before moving to Newcastle, but when Billy phones the police in a last ditch effort for more time with Lola, Jay is arrested, and Lola and Lexi move to Newcastle alone. Jay receives a suspended prison sentence and a three-month curfew. Although he intends to join her when the curfew is finished, Lola later calls Jay to end their relationship.
Jay starts a relationship with Linzi Bragg (Amy-Leigh Hickman), but they agree to take things slowly. When they plan to have sex for the first time, and she sends him a video of herself in her new underwear. However, Jay is forced to pick up Louise Mitchell (Tilly Keeper) from school after she gets into a fight. There, Jay discovers Louise was fighting with Linzi, realising that he has been dating a 14-year-old schoolgirl whose real name is Star. Linzi begs Jay not to end their relationship but he tells her to forget that it ever happened, and she reluctantly agrees. Jay then asks Louise not to tell anybody the truth. An unaware Ben texts Linzi from Jay's phone and she comes to Walford but Jay tells her to leave him alone. Linzi is comforted by Bex Fowler (Jasmine Armfield), who tells Linzi's mother, Thelma Bragg (Lorraine Stanley) about the relationship. Thelma goes to The Queen Vic, asking who has had sex with her 14-year-old daughter; in front of everyone, Bex says it is Jay. At home, Jay explains to his family that he did not have sex with Linzi and ended the relationship as soon as he found out her age, but Phil disowns Jay. Jay is arrested after Thelma calls the police. The photos that Linzi sent Jay are found on his phone, and Jay pleads guilty to save Linzi the ordeal of giving evidence in court and the images being shown. Jay is ordered to do 150 hours of unpaid work and to sign the Sex Offenders Register for five years. Phil refuses to harbour Jay after seeing him plead guilty, and when Jay leaves the house, he is cornered by members of Linzi's family and brutally beaten. He spends the night in hospital and then meets Ben, who says he has done nothing wrong and they should carry on as normal, however he is refused service in the pub. Billy allows Jay to stay with him. Pam Coker (Lin Blakley) suggests that he starts working at the undertakers, something her husband Les Coker (Roger Sloman) is not impressed with. However, Jay loses the job when they cannot afford to pay him. Jay struggles to live life as a convicted sex offender and when no one will employ him due to his conviction, he turns to drug dealing. Billy discovers this, warning him that he will start using drugs as well. When Honey finds out about the drug dealing and that Billy knew, she evicts them both. Jay tries to steal money from Honey and Billy's wedding fund, but they catch him doing so and Honey calls the police. The police alert social services and Jay decides to move out, so that Billy and Honey's children will not be taken into care.
Jay supports Ben after his boyfriend Paul Coker (Jonny Labey) is killed in a homophobic attack and helps Ben to find the attackers. The killers break into the Mitchells' house and kidnap Jay and Ben, putting them into a back of a van. Ben is rescued by his uncle Grant Mitchell (Ross Kemp) but Jay is left behind, but escapes when the police arrive. Shortly after giving a statement to the police, Jay decides to leave Walford. However, he is found at a homeless shelter by Pam, who takes him back to the Square and convinces Sharon to let him move back into the Mitchell house and Billy to give him his job back at the funeral home, from which he steals money to give to a drug dealer, Mike Rendon (Bailey Patrick). Mike demands more money, threatening to kill Jay, so Jay steals cash gifts from Lee (Danny-Boy Hatchard) and Whitney Carter's (Shona McGarty) wedding reception. Jay also steals jewellery from a corpse but when Billy's son Will Mitchell (Freddie Phillips) catches him, he returns it. Jay is unable to repay Mike, so Mike forces Jay to deal drugs. Billy finds out and sacks Jay. Phil is released from hospital does not want Jay living in his house, but later Phil promises to help Jay with his problem with Mike, telling him to report Mike to the police. Mike is arrested and Phil lets Jay stay with the Mitchells, and Billy gives him his job back. Jay confides in Billy his fear that no woman will want a relationship with him due to his name being on the sex offenders register, but Billy assures him that he will eventually find the right woman.
Phil decides to give Jay the lucrative car lot, and when Jay discovers how much it is worth, he confronts Phil. Phil says that he killed Jay's father, a homeless man who died in a fire at the car lot several years earlier, however, a phone call reveals this as a lie. Jay starts to resent Phil for the death of his apparent father. He decides to stay with his grandfather Bert but Bert returns to Walford unannounced. Bert departs again after talking to Phil and Jay.
In 2017, Ben is angry when Louise ends up in hospital after school bullies Madison and Alex pushed her into some candles, leaving her almost dead in hospital. Ben the tries to go and kill Madison and Alex but is stopped by Jay when he tells Ben that they are only fifteen. Ben then replies "I thought Louise was your sister too?" but Jay reassures Ben that Louise is his sister and Ben is his brother.
In January 2018, it is revealed that Max had blackmailed Phil into lying to Jay about his father dying and that Jase was Jay's real father after all. Max attempts to return the car lot to Jay, but he says he does not want anything from him. Jay is devastated when Abi dies. Pam returns to Walford and sacks Billy for illegally exhuming a grave and giving the company a bad reputation, so Jay is made acting manager. Jay is less than pleased when all of his roommates have one by one left the Square and he is forced into allowing Billy, Honey and their children to come and live with him to cover the rent. Jay starts to complain of feeling lonely and isolated due to many of his friends leaving and seemingly hitting a dead-end in his love life. Jay strikes up a friendship with Ruby Allen (Louisa Lytton) when she returns to Walford in September 2018 and begins to develop feelings for her. Ruby and Jay become close but Ruby is hesitant to take their relationship to the next level as she is coming to terms with having been raped earlier in the year. Jay supports Ruby through this.
Lola returns alongside Ben, and their daughter, Lexie. Jay and Lola share several kisses, before Ben reveals to Jay that Lola is engaged. Lola says that she will leave her fiance but Jay says that he cannot trust her and that they won’t get together. When Ruby returns to the Square, Jay tells her what happened with Lola and apologises. Ruby forgives him and Jay tells her he would like them to be in a relationship. Deciding she is ready to move on with her life, Ruby agrees.