William Mitchell was born on the 5th May 1959 in London to Stephen Mitchell and Janet Mitchell. He had a brother Charlie Mitchell born in about 1960. Billy's parents left him in a children's home, where he was physically abused by the staff. When his brother and sister-in-law died, Billy became guardian of their son Jamie, and vented his anger on him. When Billy's distant cousin Phil Mitchell realised what was happening, he took Jamie in and Billy learnt from his mistakes.
Billy moves to Walford, initially getting involved in criminal scams with Phil and Grant Mitchell. His relatives see him as little more than a lackey, and he is frequently told that he is an embarrassment to the Mitchell name. Billy finds an ally in Steve Owen, who uses him as an errand boy in numerous criminal activities including drug-dealing, in order to antagonise the Mitchell brothers. Billy works at Steve's club, E20, and is given a 1% share of the club for his services, but is often torn between his loyalties to Steve and his family. Billy falls for Janine Butcher in 2001, but she is only interested in his money, and when that runs out, she leaves him. Nevertheless, Billy helps her get over some psychological problems later that year, leading her to hope for reconciliation; Billy turns her down as he has fallen for Little Mo Morgan, his business partner in a cleaning company. Mo is being abused by her husband, Trevor (Alex Ferns), and it is not until Mo leaves him that Billy and Mo start a relationship. Billy proposes to Mo by spelling out 'MARRY ME' on her plate with alphabet spaghetti, and they get engaged. When Trevor discovers this, he kidnaps Mo, but is killed the same night in a gas explosion. Billy and Mo marry on Christmas Day 2002, the same day that Jamie dies of severe injuries after being knocked down by a car. Eleven months after their wedding, Little Mo is raped by Graham Foster. She later discovers she is pregnant as a result of the assault. Billy presses her to have an abortion, but Mo refuses. She leaves Walford to clear her head, and comes home with baby Freddie in tow, to see Graham convicted of rape. Billy and Little Mo reunite, but Billy confesses that he cannot love Freddie and thinks of Graham whenever he sees him, so the marriage ends.
In 2002, Billy confronts one of his abusers from the children's home he was placed in as a child, Ernie Johnson, now a pensioner and living in Walford. Billy breaks into Ernie's home to confront him over the abuse, shaming Ernie into moving away. Billy starts a relationship with newcomer Honey Edwards in November 2005, whom he manages during her spell as a model. The relationship prospers, despite initial disapproval from Honey's father Jack, who takes time to accept Billy. Honey falls pregnant in January 2006 and accepts Billy's marriage proposal. Their wedding is postponed when Honey is hospitalised; they re-schedule, but the second ceremony goes awry due to a practical joke played on Billy during his stag night. Billy's aunt Peggy then arranges a surprise wedding in September 2006. Billy and Honey just complete their vows when Honey goes into labour. She delivers a girl, Janet, but the Mitchells then discover she has Down's Syndrome. Honey rejects her baby; she goes into depression, contemplating infanticide and demands Janet be put up for adoption. Billy is opposed to this, but eventually consents after agonising over the decision. After Janet is taken into foster care, she suffers a health scare and is hospitalised, forcing Honey to acknowledge she loves her baby and Janet is brought home.
Honey falls pregnant again and William is born in November 2007. His birth is traumatic as Honey goes into labour after being caught-up in a violent siege; she is assaulted by a member of a football firm lead by Terry Bates. William seems to be stillborn at first but is resuscitated by medics. In December 2007, Billy and Honey suffer monetary problems; their landlady, Manju Patel, decides to evict them. Billy steals charity money to pay the rent and barricades them into the flat, but they are nevertheless evicted at Christmas. They rely on kindness of friends and relatives until the council rehouses them. Desperate for money, Billy takes on a job as a getaway driver for Jase Dyer, believing they are stealing money from Terry Bates, the leader of the firm responsible for hurting Honey. The job is a set-up, and Terry takes Jase's son, Jay Brown, hostage in their absence. With Billy's help, Jay escapes, but Jase is killed by Terry in the ensuing fight. Billy hides in the bathroom, too afraid to help Jase. Not realising this, Jay hails Billy a hero for trying to rescue his father. Billy breaks down and confesses his cowardice to Honey, revealing that he kept some of Jase's money, allegedly to give to now orphaned Jay. Honey is incensed; she threatens to leave Billy, but reconsiders on condition he never lies to her again. Billy does lie and keeps some of Jase's money, telling Honey he has donated it to charity. Honey subsequently ends their marriage and takes her children to live with her father, leaving Billy alone and depressed.
In October 2008, Billy visits Jay at his foster home and agrees to foster him, but problems arise in 2009 when Nick Cotton blackmails Billy, threatening to tell Jay the truth about his cowardice on the night of Jase's death. Unable to meet Nick's demands, Billy is forced to tell Jay the truth instead and Jay reacts in fury. Terry's gang try to stop Billy testifying at the upcoming trial, but despite threats, Billy testifies resulting in Terry receiving life imprisonment for murdering Jase. Billy is subsequently redeemed in the eyes of Jay; however, Jay later begins to idolise Phil, to the virtual exclusion of Billy.
When Billy stops a man from pestering Jean Slater at a club, she brands him a hero and they sleep together. Embarrassed by the fling, Billy insults Jean the following day, ending her hopes for romance. Billy later begins a more serious relationship with Julie Perkins, a fellow resident at the children's home he once lived in. Billy is shocked to discover Julie fell pregnant with his child in their teens; their son was adopted and is now a grown man. Billy attempts to find his son, Dan Pearce, but is devastated to discover he has died of liver failure and that his granddaughter, Lola Pearce, is in care. Billy visits Lola and they bond. Lola moves in with her grandparents but she proves troublesome, and when Billy continuously overrules Julie's attempts to discipline her, a rift forms between the couple, causing Julie to leave Walford. Lola's announcement that she is pregnant leads to Billy returning to criminality to provide for his great-grandchild—stealing from his employer Janine, and trading stolen goods and forgeries for Derek Branning. Lola gives birth to a baby girl, Lexi Pearce, and Billy tries his best to support the pair. Billy gets a job working for the local funeral directors Coker & Sons. He forms a close relationship with his employers, Les and Pam Coker. When Honey returns announcing her plan to move to Canada with Janet and William Billy is left distraught. In a fit of madness, he burns Janet's passport and, even though she is left angry, Honey allows Janet to stay with her father.
Lola chose to leave Walford, with Lexi, in 2015 after Billy tried to ruin her relationship with Jay. However, the two were on good terms when she left. After Lola's departure Honey returned to Walford. The two have since reconciled and plan to re-marry.
In December 2017, Billy comes up to Mick and asks for use of the upstairs bathroom, as the pub's public bathroom has a "dodgy ballcock". Mick allows him to.
Mick overhears Aidan shouting "Yes, that's the Queen Victoria pub in Albert Square" down the phone. Aidan says that he has bought Mick a present, to Mick's mocking. Aidan shouts to Whitney and asks her if he gave her a present to give to Mick yesterday. Whitney says yes, and explains she placed it under the Christmas tree upstairs. Aidan tells Mick it is a gun and that he "felt duty-bound to report it to the police". He explains he has around 10 minutes to get rid of it. Mick runs upstairs to find it, but can't. He asks Aidan desperately to tell him where it is. Aidan gives him a clue, and he finds it just as the police arrive. He hands it to Billy, who is returning from his toilet trip, and tells him to leave and give it to Phil. Billy does so.
When Billy gives Phil the gun, Phil jumps back, thinking Billy is going to shoot him, but he takes it from a trembling Billy.
Billy discovers Adam Bateman is having an affair with Habiba Ahmed and attempts to tell Honey Mitchell, whom he is still in love with. Honey yells at Billy and accuses him of trying to destroy her and Adam's relationship because he is jealous. Billy also informs Mariam and Arshad about the affair but Arshad orders him to get out.
Billy later hires Karen Taylor as a bar tender however he soon realizes that Karen is not really cut out for the job. Billy then discovers that Karen is in trouble with Ben and tries to help sort it.