Mo, who previously lived in Walford, returns there after almost forty years away, with her son-in-law, Charlie Slater (Derek Martin) and granddaughters, Lynne Hobbs (Elaine Lordan), Kat Slater (Jessie Wallace), Little Mo Morgan (Kacey Ainsworth) and Zoe Slater (Michelle Ryan). Mo's rivalry with her former best friend Pat Butcher (Pam St. Clement) arises, but they reconcile at Mo's former husband and Pat's brother, Jimmy Harris's (Alex King) grave. Much of Mo's time is spent selling poor quality stolen goods and she trades with Fat Elvis (Shenton Dixon). She is assisted by Mickey (Joe Swash) and Darren Miller (Charlie G. Hawkins) and for a while sets up a sex chatline. She also works with Charlie's great-niece, Stacey (Lacey Turner), on her market stall, selling clothes. She has a brief relationship with Bert Atkinson (Dave Hill), but ends it because Bert is also seeing her friend, Ina Foot (Ina Clare). They reconcile, but break up again when Bert's ex-wife, Evie (Marji Campi), arrives in Walford, terminally ill. Mo begins selling cigarettes that she bought in Spain and two of her customers turn out to be Revenue and Customs officers. She is sentenced to 100 hours of community service, for tax evasion. After Stacey marries Bradley Branning (Charlie Clements), Mo tries to sell their wedding gifts.
Mo sees Charlie's partner, Brenda Boyle (Carmel Cryan), embracing another man at the tube station, but the man is Brenda's brother, Clive Robinson (Col Farrel). Brenda reveals to Mo that she is moving to Madeira with her family and Mo convinces Charlie not to let her go. However, Brenda and Charlie leave together, leaving Mo devastated. She begins drinking heavily and protests against the sale of Charlie's taxi. Charlie returns to Walford with a woman called Orlenda (Mary Tamm), but Mo does not trust her. Orlenda leaves, after admitting she was scamming Charlie, but returns his money. Mo takes charge of a children's nativity play, but fails to keep the children under control. In a raffle, she wins Max Branning's (Jake Wood) car, but sells it, to Max's dismay. When Archie Mitchell (Larry Lamb) is murdered, Mo starts a betting circle on who killed him. She runs when she sees a police officer, but later, another officer appears behind her and she asks him if he wants to place a bet. She goes on holiday to Spain with Fat Elvis and when she returns, she is surprised to discover that Bradley is dead. Mo attempts to sell Lucas Johnson's (Don Gilet) bibles to the public, as gruesome souvenirs of the killer. Her insensitivity gets her thrown out of The Queen Victoria public house (The Vic). Mo receives a phone call from Kat, who wants Zoe's birth certificate. Mo meets Kat at a café and gives her the certificate. Kat admits to Mo that she conned a man and needs a new identity. Kat also lies to Mo, claiming that her husband, Alfie Moon (Shane Ritchie), is dead. Mo persuades Kat to come home and Kat reluctantly agrees, but when they get home, men are waiting for Kat and her money. However, Alfie sends them away. Mo is shocked to learn that Kat is pregnant and Alfie is not the father. When Kat goes into labour, Mo delivers her son, Tommy Moon, on the floor of The Vic's barrel store. On the same day, Ronnie Branning (Samantha Womack) gives birth to a baby boy, James Branning. On New Year's Eve, Ronnie finds that James has died and she swaps him with Tommy. Alfie and the family find the deceased baby and believe it is Tommy.
After Kat blames Charlie for Tommy's death, Charlie decides to go and live with Lynne, despite Mo begging him not to go. Left living alone, Mo asks Kat if she can move into The Vic and let No. 23. Kat refuses initially, but later agrees when Mo reminds her that she never complained about living with Kat's bad habits. Mo continues with her dodgy deals, while staying at the pub, including persuading Jean Slater (Gillian Wright) to serve "turkey" for pub lunches, which turns out to be swan meat. When a benefit fraud officer declares that Jean has been illegally claiming benefits while working at the pub, Mo admits to Jean and Kat that she has been claiming benefits in Jean's name. Jean agrees to take the blame, as Mo has previous convictions. When Mo learns that her old enemy, Pat, is terminally ill, she visits her in hospital and they end their long-standing feud. When Pat later dies, Mo returns Pat's Miss Butlin's sash, that she stole years ago, out of jealousy. When Mo learns that Charlie has suffered a stroke, she leaves Walford to look after him. Several months later, Jean visits them and Mo returns to The Vic when she believes that Jean is hiding something from her. Her suspicions are confirmed, when she discovers that Alfie is allowing Roxy Mitchell (Rita Simons) to stay in The Vic. Mo continues to disapprove of Roxy's presence in The Vic, after Kat and Alfie split up. It is later revealed that Mo and Charlie have rented of a villa in Devon, owned by "a mate of Mo's".
Mo returns to collect rent from Kat, Michael Moon (Steve John Shepherd) and Alice Branning (Jasmyn Banks). She demands Alice pays by the end of the day, or she will evict her. Kat later convinces Mo to give up her keys to the house. Three months later, Jean discovers that Mo is stealing vegetables from the Walford allotments and Mo convinces Jean's new partner, Ollie Walters (Tony O'Callaghan), a police officer whom Mo knows, not to report her, as she is Jean's family. At Christmas, Mo and Charlie visit Walford, where it is revealed that Kat is pregnant with twins. Two months later, Alfie calls Mo, after dealing with some serious family issues, raised by the re-appearance of Stacey after a four-year absence, to which Mo plans to spend more time in Walford to support her family. Mo is delighted when Kat and Alfie's twins, Bert and Ernie Moon (Freddie and Stanley Beale), are born, but the family's joy is cut short when a fire starts at their house and when it reaches a supply of knock-off hairspray, which Mo had intended to sell, a huge explosion destroys everything that they own. Kat is badly injured and Mo is wracked with guilt, briefly moving in with Masood Ahmed (Nitin Ganatra), before taking an extended holiday in Lanzarote with Charlie.
Mo returns to Albert Square and is shocked to discover that Alfie and Kat have, once again, separated and that her family are struggling, financially. Mo reveals to a destitute Kat that she inherited money from her uncle, Harry, who had raped her as a child. Kat turns the money down, so Mo secretly has the money transferred to her own account, pretending to be Kat. When Kat discovers this, she is livid and burns the money in the street. Kat and Alfie reconcile and after winning the jackpot on a scratch card, plan to move to Tenerife. At their leaving party, Mo shows a stunned Kat an engagement ring, revealing that she has accepted a marriage proposal from her longtime business associate and romantic interest, Fat Elvis. Shortly after Kat and Alfie's departure, Mo and Fat Elvis leave for Greece. When Mo returns alongside Charlie, she reveals that Fat Elvis has left her. When Kat learns she had also given birth to a boy along with Zoe, she realises that Mo knew about the secret, so angrily demands the truth from her. During the argument with Mo that ensues, Charlie suffers a heart attack in front of Mo, Stacey and Kat. After suffering from a cardiac arrest, Charlie dies, which devastates the family. After attending Charlie's funeral, Mo moves to Kent to live with her granddaughter Belinda Peacock (Carli Norris), as Kat cannot forgive her for having her son adopted. When Belinda moves to Walford after her marriage ends, Mo's whereabouts are not mentioned.
The character of Mo appears in two spin-offs of EastEnders. She features in the 2004 'soap bubble' EastEnders: Pat and Mo, which explores her past with Pat through flashbacks, in which she is played by Lorraine Stanley. The episode was broadcast to give the viewer "and understanding of, and history to [the] characters". The episode is partly set in the 1950s and explains Pat and Mo's dislike for each other. A show source said, "When the Slaters arrived in the square three years ago, Pat was horrified. Now the time has come for them to bury the hatchet—or at least try to." Mo also appears in the straight-to-video spin-off EastEnders: Slaters in Detention in 2003, and in the 2010 internet spin-off EastEnders: E20.
In 2000, EastEnders' executive producer John Yorke decided to introduce the "classic" Slater family. He felt the show needed to go back to its roots and bring back some traditional values. BBC's head of drama, Mal Young commented, "We do not have enough solid families in the soap, there were a lot of fractured families and people who were alone." The family were created as a replacement for the di Marco family, who were axed by Yorke. The family consisted of, grandmother Mo (Laila Morse), father Charlie (Derek Martin), his four children, Lynne (Elaine Lordan), Kat Slater (Jessie Wallace), Little Mo (Kacey Ainsworth), Zoe (Michelle Ryan), as well as Lynne's boyfriend Garry Hobbs (Ricky Groves).
The character of Mo and the rest of the Slater family were created during workshops with Tony Jordan and John Yorke. London actors were invited to improvise in groups, and the characters were created during these improvisations. According to the BBC's head of drama series, Mal Young, the family was brought in as the serial did not "have enough solid families in the soap, there were a lot of fractured families and people who were alone." Laila Morse was cast as Mo, and made her first on-screen appearance on 18 September 2000. She was named "Arrival of the Year" by TV critic Ian Hyland in 2000.
Mo has been described as "a very strong person [who] loves the family [and] wouldn't have anything said about them" by Morse. She has been said to "hold the family together … [acting] as a surrogate mum to her granddaughters". Her criminal activity has led to her being described as a "female Fagin or Del Boy"and the "queen of knock-off". Mo has also been described as a "tough cookie" and a "fighter". Another writer from The People said that Mo is "tough-talking".
Mo has been described by Ian Hyland from the Sunday Mirror as a "battleaxe" and a woman "with a face like a bulldog chewing a whole nest of wasps and a mouth like a foghorn in the Grand Canyon, she's the only woman I've ever seen who makes Fat Pat look demure." He added that "This time they've bought finest East End. Every time Mo, the granny, opens her mouth I think jellied eels are going to fly out." Another writer for the Sunday Mirror, Paul Scott said: "As Big Mo Slater she is the battleaxe from Hell whose tongue-lashings have even the hardest of Walford's hard cases running for cover." Charlie Catchpole from the Daily Mirror said of Mo's arrival: "I'm sure, that no sooner does a Romanian weightlifter go missing from the Olympics than Mighty Mo Harris arrives in Albert Square." John Russell from The People called Mo a "motormouth".