Having believed that she was infertile, Sharon discovers that she is pregnant on Christmas Day 2005. Dennis is stabbed and killed in the early hours of New Year's Day 2006, and Sharon moves to Florida, where she gives birth to Dennis Jr. In July 2006, Pauline Fowler receives the news that Sharon has given birth to her son.
Dennis appears in August 2012, on the day of Sharon's wedding, to her partner, John Hewland. Sharon discovers that he is missing, and eventually realises that John has kidnapped him. Sharon asks her ex-lover Phil Mitchell, to help rescue Dennis, and he takes the pair back to Walford. Dennis starts Walford Primary School, and starts to think of Phil as a father-figure. Dennis is upset when Phil's enemy Derek Branning reveals the truth about his father's murder, as he believed that he was a helicopter pilot, who died saving others. Sharon admits to Dennis that she lied to protect him. The revelation upsets and enrages Dennis, though he later forgives his mother. In 2013, after losing their home at the local bed and breakfast, they move in with Derek's older brother, Jack. Jack teaches Dennis to box, leading to him starting a fight at school.
In 2014, Sharon is attacked in her bar and as a result, she loses her temper and shakes Dennis, who is then frightened and runs away. In early 2015, a fiery clash leads Dennis to leave scratches on his mother's face. When he misbehaves in school, and around Sharon's friends when she is not there, Sharon refuses to believe that Dennis is lying about his behaviour, and blames those that he has upset when Phil's stepson Ian Beale and his wife inform her over Dennis's misbehaviour.
On Christmas Day 2015, after overhearing Sharon and Phil during dinner, Dennis tells his best-friend and Ian's son Bobby, that he knows he killed his half-sister, Lucy. When Dennis and Bobby race their bikes, they fight on the Walford Common and Phil and Ian find Dennis. Phil is drunk whilst driving and crashes the car while distracted. Dennis is severely injured and taken to hospital. He has a subdural haematoma which requires an operation. Ian takes responsibility by saying he was driving, making a deal with Phil. However, Phil confesses that he was the one who drove the car, so when Dennis is discharged from the hospital, Sharon takes him away to stay with her father Gavin Sullivan. Dennis returns a week and a half later with Sharon for Billy and Honey Mitchell's second engagement party, only to find Phil on the floor, having collapsed from coughing up blood. Sharon reveals to Kathy Sullivan that Gavin hit Dennis during their stay and that is the reason why they left. However, in July 2016, Dennis misses Gavin and has a second mobile phone to which he only contacts Gavin from. Sharon discovers this and is furious, but is later unnerved when Gavin collects Dennis from school, and realises that he could easily abduct Dennis.
In November 2016, Sharon becomes concerned about Dennis's behaviour and decides to apply to send him to an alternative high school. Dennis takes the death of a school friend, Eddie Kendall, hard and confides in his stepsister, Louise, that he does not want Phil to die, who has rejected a transplant. Phil overhears this and announces to his family he is going on the transplant list. Sharon decides to let Dennis go to Eddie's funeral. On the day of Eddie's funeral, Dennis is unsure about going, but Phil encourages him so he does not regret it and can say goodbye. Dennis later attends the funeral. In December 2016, Dennis gets a part in the Christmas show, and volunteers to take a part that involves his and Louise's characters kissing, saying he would not mind it, but the part is given to someone else. He is later reprimanded when he gets caught watching Louise and other girls getting changed. Sharon punishes Dennis for this but Phil says he was doing what normal boys do.
While Sharon is on holiday with Phil, Sharon's best friend, Michelle Fowler, looks after Dennis and Louise. In March 2017, Dennis sees Michelle and 17-year-old Preston Cooper kissing so he starts blackmailing Michelle into buying him things and letting him stay off school. Michelle is furious when she finds out Dennis bought a video game using her credit card and Dennis brands her a paedophile, resulting in a slap from Michelle. Louise does not witness this but catches Michelle calling Dennis a "brat". Dennis demands £100 from Michelle to keep quiet about her hitting him, but she refuses and he kicks her. When Louise finds him, she tries to get through to him over arguing with Michelle and he reveals that Michelle and Preston are having an affair. The affair is revealed which leads to Michelle crashing Phil's car and Sharon returning and convincing Preston to return to the United States in April 2017. Sharon then tells Dennis and Louise to be nice to Michelle. When Louise has a party with her friends, Madison Drake and Alexandra D'Costa, Dennis turns up and Louise lets him stay, on the condition he keeps quiet about them having alcohol. Louise gets drunk after her drink is spiked and is sick, and as she staggers outside, she coughs up blood and collapses, which panics Dennis and those around Louise. In July 2017, Dennis and Will Mitchell plan to throw eggs at Keegan Baker, but accidentally hit his half-sister, Bernadette. Keegan gets into Dennis's house and tells them Bernadette is pregnant. Dennis and Will apologise and Keegan attempts to force Dennis to smoke a cigarette, but is caught and thrown out by Michelle. After this, Dennis forgives Michelle for hitting him. In August 2017, Sharon and Phil receive a date for Dennis's adoption hearing. Dennis asks Phil if he wants him to change his surname to Mitchell upon his adoption as he wants to keep his current surname, which Phil does not mind.
In 2018, Stuart Highway gets Dennis to steal the keys to the Vic as part of his plan to take revenge on the Carter family. Dennis wants a new phone but Sharon refuses so he buys a stolen phone from Jagger Rawley and he then hides a bag of phones in the garage for him, which Keanu Taylor finds. Phil sees off Jagger's gang when they arrive and they later threaten Dennis via text message. Keanu confronts the gang and they attack them both, though Phil sees them off again. The gang later kidnap Dennis but he is rescued by Keanu. Stuart gives Dennis a bag of weapons before he breaks into the Vic again and is shot. Dennis is shocked when he hears Stuart has been shot and worries that the police are looking for him, telling Sharon that he heard the gunshot.
On 19th February 2020, the Queen Victoria had won the best pub and was rewarded with a boat trip on the River Thames for all members of Albert Square. After Ian found out that Dennis was responsible for putting his son in hospital, he trapped him in a room on the lower deck of the boat. The boat crashed, caused by Phil Mitchell and Keanu Taylor and Ian tried to save Dennis from drowning. However, The Boat Rocked and made Ian drop the keys into the water, unable to be reached by Ian. Following this, Ian broke the door down with a fire extinguisher and got Dennis out. As they were about to go up the steps, a wave of water flooded the lower decks completely, Making Ian's hand slip and lose Dennis. Ian tried to find Dennis but when they got to shore, Denny died from a cardiac arrest following his Injuries. Whilst this was occuring, his mother, Sharon Mitchell gave birth to his brother, Kayden Watts. Keanu delivered the news to Sharon about Dennis, leading to Keanu's departure because of Sharon's accusation (Manslaughter of Dennis) against Keanu.