When the Carters move to Albert Square, Lee is serving in the Army and speaks with his mother Linda using the internet. Three months later, his father Mick visits Stan Carter, Mick's grandfather but before he can reach the flat he is punched by an unknown youth, and afterwards Mick discloses to Max Branning that he believes that the man was his son, Lee. A week later, it is Lee who saves Stan from a small fire at The Queen Victoria public house. The family meet at the hospital. When Mick asks him why he punched him, he confirms that he was in Stan's flat. Lee then has sex with Lucy Beale, however she insists it was a one-night stand. He begins pursuing Whitney Dean instead, but later resumes his affair with Lucy. When Lucy is found dead after being murdered, the police take a DNA sample from Lee, before he returns to the army.
He returns as a surprise for Linda on her birthday. He bonds with his uncle, Dean Wicks, and becomes drunk at his mother's party. After passing out on Mick's bed, he tells his father he did something stupid in connection with Lucy. The next day, Lee reveals that he saw Lucy arguing with Billy Mitchell. Lee, along with his great aunt Tina Carter and her girlfriend Tosh Mackintosh, confront Billy, but he angrily tells them to leave. In Dot Branning's memorial to the commemorate the centenary of World War I, Lee raises and lowers the Union Flag as a memorial. He tries to win back Whitney, but she is still upset about how he abandoned her for Lucy. Eventually, she forgives him, and they begin a relationship. Whitney's adoptive mother Bianca Butcher protests to the relationship until Lee wins her round. Lee and Whitney have sex, and she confides in him that her estranged brother Ryan Malloy is trying to get back in contact with her.
Lee is later unhappy when Ben Mitchell returns to Walford after serving time in prison for killing his father Phil Mitchell's ex-girlfriend Shirley Carter's best friend, Heather Trott. Lee's brother Johnny Carter shows an interest in him, knowing that he is gay, but Ben tells him he is no longer homosexual and insults Johnny with homophobic names, resulting in Lee viciously attacking him. Lee realises that Lauren Branning's alibi is suspicious when he views her outside Lucy's house on a video taken on the night of Lucy's death. He also later shares this information with the police investigating Lucy's murder. Lee and Nancy Carter later learn that Mick and Linda are not actually married as they had previously thought.
During Christmas and the New Year after Johnny's departure to go travelling, Lee and Nancy become paranoid about the situation between their parents and Dean, who believe that Dean has done something wrong. Lee and Nancy are left in charge of The Queen Vic for several weeks while Mick and Linda go on holiday. Nancy is determined to know what Dean has done, but Shirley and Tina refuse to talk to Nancy without Mick and Linda's consent. When Shirley moves Dean in, Nancy is upset by this and Lee tries to talk Nancy into accepting Dean's presence while Mick and Linda are away. After throwing a party, Mick and Linda later return and are horrified to see Shirley behind the bar and Dean drinking in The Queen Vic. After reporting Dean to the police, Linda finally reveals to Lee and Nancy that Dean raped her and it is possible that he could be the father of Linda's unborn baby. Lee is angry to learn of this news and is determined to make Dean pay. Both Lee and Mick's tempers worsen when Dean continues to harass them and their family. Lee and Dean fight in the street when Dean denies attacking Linda and Mick makes it clear to Dean and Shirley to stay away from The Queen Vic, although Shirley decides to instead stand by him when Dean disappears for a few weeks. Lee is later happy when Linda proposes to Mick, who accepts.
Lee and his family are upset to learn that Stan has died after he was diagnosed with terminal prostate cancer. The feud between Dean, Shirley and Mick continues, which begins to cause disruption for the other members of the Carter family. Dean is arrested on suspicion of breaking his bail conditions, but he is later released from prison. Linda gives birth to a baby boy, later named Ollie, after falling down the stairs and Lee is struggling to come to terms with the possibility that the baby may only be his half-brother. Mick talks Lee round to accepting the fact that the baby may be Dean's but he will always be the baby's brother, half or not. Lee is later relieved when he learns that Ollie is not Dean's, and visits Linda and Ollie in hospital, with the company of Nancy and Whitney.
Lee begins to develop clinical depression, which leads to him missing work and being discharged from the Army when he punches one of his colleagues who flirts with Whitney. Lee is prescribed medication for his condition but he refuses take it. He gets a job at the local car lot, but Lee develops bitter feelings towards those around him, repeatedly shunning Whitney and his family. His worsening alcoholism nearly gets him sacked from the car lot, but Whitney convinces Fatboy (Ricky Norwood) to give Lee one more chance. Lee's mood improves when he makes a sale at the car lot and is invited to a Halloween party. However, a heavily pregnant Stacey Branning is electrocuted by a broken socket that Lee did not fix properly. Stacey's boyfriend Martin Fowler punches Lee and threatens to kill him if Stacey's baby dies. Lee breaks down in Whitney's arms when he realises the baby could be dead because of him and confesses to being drunk when fixing the socket, but is relieved when Whitney tells him that the baby and Stacey are not badly injured. He and Whitney are later invited to have lunch with Martin, Stacey, Kush Kazemi and Shabnam Masood in The Queen Vic, where Martin apologises to Lee for punching him and forgives him for the incident. Days later, during Dean and Roxy Mitchell's engagement party, Lee gets drunk and confronts Shirley after finding out that she and Mick have been seeing each other secretly. During the argument, Lee tries to attack his grandfather Buster Briggs, who escorts him out and takes him to the café. Buster tells Lee that he needs to start taking his anti-depressants and stay focused on his career. He then returns him home and Lee tells Mick that he has no problem with him and Shirley seeing each other. Lee later knocks himself unconscious while trying to eject Dean from the pub, so is carried upstairs by Mick and Whitney. When he regains consciousness, he overhears Whitney confess to Mick via the baby monitor that she intends to break up with him after Mick and Linda's upcoming wedding, unable to handle his depression. On New Year's Day 2016, the day of the wedding, Lee goes missing and Whitney and Mick find him drunk at the park. Lee then tells Whitney that he blames himself for her wanting to break up with him, and she promises him that they will work things out. Dean turns up to the wedding after attempting to rape Roxy and pushes Shirley into a nearby lake, and Mick saves them both. The police subsequently arrive and arrest Dean for the attempted rape of Roxy. Later, when Phil turns up at The Queen Vic drunk and demands more drinks, Lee tries to escort him out but Phil, being the owner of the car lot, sacks him. Lee later works alongside Buster at his local fish stall, but sneaks off one afternoon for an interview with local landlady Thelma Bragg, whom was initially interested in hiring Nancy. Lee gets the job, but when Nancy, who was covering his shift for Buster, finds out, she calls him a spineless traitor before assuring him that he will not last long given his history of failure. Although Whitney praises Lee for getting the job, he walks away in misery.
To assist with getting to work, Mick buys Lee a new car, which he takes Whitney for a drive in. During their drive, Whitney confesses to kissing Mick. An enraged Lee finds Mick and Linda at Ian Beale's restaurant and tries to attack them both. He later gets drunk and is thrown out of The Albert by Vincent Hubbard for insulting his wife, Kim Fox-Hubbard (Tameka Empson). He then bumps into an equally drunk Abi Branning and they end up having unprotected sex. Whitney goes to stay with Sonia Fowler, where the next day, Nancy drops the rest of Whitney's belongings off as Lee is unable to face her after what happened with Mick. However, he later sees Abi talking to Whitney, and fears she may have told them about their one-night stand. Abi denies doing so, but tells Lee that Whitney is planning to leave Walford. Lee rushes to Sonia's house and begs Whitney to stay, finally confessing that he has not been taking his anti-depressants. When his family finds out, Mick furiously denounces him for lying, but Lee promises to ensure he gets better from now on. Later, when Whitney comes down to The Queen Vic after talking to Nancy, Lee asks her to marry him, and she accepts. Lee's relationship with Nancy is further tarnished when he reveals he told Thelma about Nancy's epilepsy, which further jeopardised her chances of getting the job. She then leaves The Queen Vic to move in with Donna Yates and works on her market stall. Lee eventually quits his new job following what he believes to be sexual harassment from Thelma, and decides to work at The Queen Vic, taking Nancy's old bartending job. Nancy joins everyone for a Mother's Day meal, but when she finds out Lee took her old job, they have another argument. Lee tells her that he is not surprised her boyfriend Tamwar Masood left her, prompting Nancy to angrily push Lee. He falls into Ollie's highchair, sending him and Ollie crashing to the floor as Nancy and Whitney watch in shock. Mick and Linda rush back to find that Ollie has no apparent injuries, but Nancy and Lee continue to argue despite Mick and Linda's protests. Eventually, Mick loses his temper, smashing a chair and throwing Lee and Nancy out of The Queen Vic. When Linda later finds Ollie not breathing in his cot and later suffers a seizure, the Carters are told that Ollie may have brain damage, after hitting his head severely during Lee and Nancy's fight. Mick blames Nancy for the accident, and after months of feuding, he forgives her, however Nancy and Tamwar leave Walford to go travelling.
When Whitney discovers she is pregnant, Lee worries that he would not be a good father due to his depression. They visit the doctors, but both Lee and Whitney are tested positive for chlamydia, which Lee caught from Abi. Lee lets Whitney wrongly believe that she had caught the STI during her past relationship with Rob Grayson, who sexually exploited her. Abi confesses to Whitney about the one-night stand and she ends her relationship with Lee and considers leaving Walford, but after speaking to Linda, she decides to give Lee another chance, saying that having a baby with him would bring her the happiness that she needs. To make up for his mistakes, Lee takes Whitney to the registry office to book their wedding date. Two months later, Lee and Whitney are devastated when Whitney has suffers a miscarriage. Lee is agitated when he believes that him giving Whitney chlamydia may have caused the miscarriage. He cancels his stag party and promises Whitney that he will change and be a better boyfriend, only for her to tell him that she is moving to Milton Keynes to stay with Bianca. Lee is accepting of this and after Whitney reassures him that their relationship is not over, he watches her leave tearfully.However, Whitney returns the next day, admitting that she missed Lee. Lee tells her how much he wants to look after her and she tells him that she is sure that he will. Lee gets a job, telling everyone he is a sales executive in the city. However, he worries about how he will cope. After his first day at work, Lee uses a credit card to buy Whitney an expensive pair of earrings. Johnny worries about the amount of money that Lee is spending on his wedding, but Lee insists he is fine. Lee checks his bank account and sees he is £250 overdrawn, and then applies for a £2000 payday loan. Lee realises he should be honest with Whitney about the money but he backs out of telling her. When Abi realises Lee is not over Whitney's miscarriage, she tells him she should have told him about the chlamydia as she read that it could have caused the miscarriage, leaving him worried. Lee confronts Whitney over her miscarriage lie, and she admits that she worried how he would cope but now wants complete honesty, to which he agrees. However, he ignores reminders about repaying the loan. Johnny discovers Lee's debts and tells him to be honest with Whitney before they marry the following day. He attempts to tell her but she thinks he is stressed over not spending time together and tells him not to worry. Lee calls in sick to work so that he and Whitney can marry. On the wedding night, Lee pretends to be asleep when Whitney wants to consummate the marriage.
It is revealed that Lee has lied about his job and he is working in a call centre and is the worst performer, and his boss realises, he called in sick for his wedding, so docks his pay. Trapped in a depressed state, Lee cries at home but keeps the truth from his family. When Whitney surprises Lee at work, she assumes Lee's colleague Oz Bolat is his employee. Whitney hopes to move to an expensive flat in Stratford and asks Lee to put a deposit on the flat but knowing he cannot afford it, he does not do so and tells her he was too late. However, Jack Branning offers one of his flats on Albert Square at a reduced rate so Whitney is happy but Lee still knows he cannot afford it. He asks his boss for an advance on his wages but is refused due to his poor performance. Lee then steals a sentimental piece of jewellery from Linda, attempting to pawn it, but he is not offered what he expected. He tells Shirley he cannot afford a flat, so she tells him that Whitney will be happy as long as she has him. He cheers up following Shirley's comment, but when Whitney buys a picture reading "home sweet home" for the flat, he steals charity money from the pub and tells Jack he has a deposit. Whitney and Lee move in but he makes excuses not to have sex with her and she worries when his mood swings return. When Whitney seduces Lee, he suffers erectile dysfunction due to his depression, though Whitney comforts him. Whitney then suspects Lee is having an affair, and when she tries to surprise him by visiting him at work, she finds out he has called in sick. He spends the day hiding in the allotments, briefly encountering Patrick Trueman, who tells him that relationships built on lies never last. Whitney confronts Lee, accusing him of having an affair, but he says he was buying her a Christmas present. Whitney believes him and they finally have sex. At work, Lee is continuously harassed and bullied by his aggressive colleague Oz, who tells him he knows he was not sick the previous day and can prove it. Lee pushes Oz, but later apologises, saying it will not happen again. Oz agrees, revealing that he has slashed Lee's suit jacket.
A few nights later, The Queen Vic is violently burgled by three masked men armed with baseball bats, who steal thousands of pounds of money and almost abduct Ollie during their thievery, while also striking Johnny across the face with a baseball bat. It is later revealed that the culprits are Oz and Lee's other colleagues and that Lee organised the robbery. However, Oz denies Lee his share of the money as he was supposed to make sure Whitney was not there as she could have recognised Oz, seeing this mistake as botching up the operation. Lee cries when he causes a paranoid Linda to jump. Whitney decides to host a Carter family Christmas lunch at her and Lee's home and refuses any financial contribution from Linda as Lee is doing well at work; when Lee hears the plan, he tries to secure another payday loan. When Whitney buys an expensive Christmas gift for Linda, Lee applies for a further loan but is refused. A package is delivered for his neighbour, which Lee opens to find a virtual reality headset, which he pawns for cash. On Christmas Day, Lee is arrested for conspiracy to commit robbery. The next day he is released without charge and tells Whitney and Mick that it was mistaken identity, but when Whitney angrily demands the truth, he admits that he told his colleagues that cash was kept in the pub overnight because he wanted to be liked and accepted. She calls him pathetic and says he disgusts her but says she will not tell Mick because he is dealing with the stress of Linda being in Spain with her mother Elaine Peacock, who has suffered a stroke.
Lee asks Whitney to take a pregnancy test but she says she cannot get over that he was involved in the robbery, calling him "spineless". Lee is frustrated when he is late for work, crying and shouting at a parking attendant, Karen Beckworth, when a machine will not take his money. At work, Lee gets a sale, but Oz says the old woman he convinced to change energy supplier will lose money and could die when she finds out. Lee passes his review with Haroon, who tells him to "man up" in 2017 and Lee then sees a fitness poster reading, "2017, time to be a man". Oz catches Lee crying and calls him a "cry-baby girl", trying to take a photo on his phone and telling Lee that he would lose everything if his family and friends knew the truth, calling him "spineless", and Whitney would leave if she met a "real man". Lee goes to his car where he vents his frustrations, writes a note saying "I'm sorry", and leaves his car, heading for the edge of the roof of the car park where he contemplates suicide. Karen finds him; he says that unlike other people, he finds life hard and is pretending all the time, feels worthless and has seriously let his family down; Karen tells him he is not a bad person, and tells him life has its ups and downs, convincing him to come down and go to her office where she gives him a card from the Samaritans; after he leaves, she cries in front of a photograph of a boy. At home, Whitney tells Lee she is not pregnant but she wants to be, and he says they will take things one step at a time.
Mick finds the Samaritans card in Lee's phone case and confronts him about it, asking why he would confide in a stranger and not his family. Lee finally confesses that he was in on the robbery of the Vic alongside his colleagues, before also admitting he had thought about suicide because of how bad he felt about the robbery. An enraged Mick tells Lee he should have gone through with the suicide, but later says he did not mean it but admits that he cannot forgive Lee, who breaks down in tears. Lee offers to accompany Mick to Ollie's next appointment but Mick refuses. He then tells Lee that while he is still not ready to forgive him, he will lend him money to clear his debt for Whitney's sake on the condition that Lee tells Whitney the truth about his job. When Lee tells Whitney the truth, she is devastated to learn that Lee lied because he thought she would be disappointed due to the lack of money his real job brings, but chooses to forgive him. After she tells Mick this, Mick forces Lee to give him Oz's address and returns with blood on his knuckles, sparking concern from Babe. When Lee next sees Oz, he has a black eye and tells Lee they should keep away from each other from now on.
Lee and Whitney begin to argue over money and he blames Whitney for his financial difficulties. This causes much tension for the couple and he becomes increasingly jealous when Danny Mitchell tries to kiss her. Whitney decides to stay at The Vic and Lee apologises to Whitney the next day for his behaviour. Babe gets the Carters into trouble with the law and when everyone turns against her, she reveals Lee's involvement in the robbery; resulting Johnny to become hostile to Lee. Whitney decides to pawn her wedding rings so she and Lee can afford a night out. She invites one of Lee's ex-army friends ("Beanbag") and persuades him to offer Lee a job in security after informing them that he is in Dover doing such a job. Lee thinks that Whitney is flirting with his friend which causes a row. When Whitney admits that she pawned her wedding ring, she tells Lee that she wishes that he was more like Mick. Lee hits her in a rage of anger and is instantly wracked with guilt. He begs Whitney for forgiveness, but she lays on their bed in silence clearly frightened of Lee. The next day, Whitney decides to forgive Lee on the condition that he will never assault her again, therefore he should seek anger management therapy. Lee gratefully accepts for the sake of working out their marriage.
On Valentine's Day, Mick tells Lee that the best thing he can do for Whitney is to leave her. Despite this, on his way to work, Lee talks to Patrick Trueman, who gives him advice on his marriage. Lee then treats Whitney to a romantic dinner, and just after asking for the bill, Lee tells Whitney that he has left a surprise for her back at the flat and insists he should go back. While waiting for him to return, Whitney realises that Lee has gone for a long time, so she returns to their flat, only to find all of his belongings are gone and reads a handwritten note from Lee saying he no longer loves her. Whitney attempts to contact him, to no avail, not knowing that Lee has called Mick requesting to see him. Mick meets Lee, who tells him that he is going to remain in Dover because he cannot stay in Walford. He returns to Walford briefly to see Whitney and tearfully explains the situation. An upset Whitney, realising that Lee plans to leave on his own to figure himself out, tells him to leave and she watches him from the window as he drives off. After saying a tearful farewell to his family, Lee leaves after Mick tells him that Walford will always be his home.
Several weeks later, Lee attempts to call Whitney but the pub's new manager Woody Woodward tells Lee that she does not want to speak to him, so Lee sends his friend Moose to see her, who says that Lee wants a divorce. Whitney receives a letter from Lee's solicitor, citing her unreasonable behaviour as grounds for the divorce, but Woody tells her it is not a big deal and Whitney agrees, deciding it is time to move on from Lee.
Over two-and-a-half years later, during Christmas, Mick contacts Lee and asks him to come back to Walford to surprise Linda. However, he returns to find her in a drunken, unconscious state on the ground. Lee rushes her mother to hospital and questions her and Mick repeatedly about her behaviour. He also confesses that he is in a relationship with a woman named Sophie. Linda denies having a drink problem. Lee soon discovers that Mick has been having panic attacks and is shocked to see the animosity between his parents. He admits to Linda that he once tried to commit suicide as he felt he could not talk to anyone, and he urges her to talk to Mick. Meanwhile, Whitney is angry with Lee for not talking to her about their divorce and blaming her for the breakdown of their marriage. Lee apologises to her, saying that she did not deserve the way he treated her. Whitney then admits that she is scared to leave her house, due to being stalked by Leo King. Lee advises her to fight back. Buoyed by Lee's advice, Whitney dresses up for New Year's Eve and Lee is proud of her and they agree to be friends. On New Year's Eve, Linda becomes unhealthily drunk and is even disorderly with Mick and Lee. Lee departs the Square once again after advising Linda to stop arguing with Mick.
Lee made a return to the show on the 31st of December telling his dad to tell his mom how he really feels.