Mandy Maguire was the only daughter of the Maguires, Chatsworth's local crime family and the Gallagher Family's neighbours. She is the mother of Lip's daughter, Katie. She was generally more law-abiding than the rest of her family, but was fiercely loyal to them and would often have a violent and abusive streak. Her brothers are the briefly seen Donny and Joey, secretly gay Micky, Shane and Jamie, as well as the late Fergal. Her half-sister, Cilla was born after her death. Mandy died tragically in Series 6 during a gas explosion caused by the psychotic Maureen in her attempt for revenge on Mandy's father, Paddy.
She was briefly seen in Series 1, where she developed a crush on Ian Gallagher after he apparently stuck up for her against bullies at school. She invited herself round to his house and tried to force him into having sex with her, but failed due to Ian's secret homosexuality. She left, distraught and emotional, and her brothers formed a lynch mob in response, hunting Ian down and essentially turning him into a fugitive. He approached her one morning and despite her aggression, forced her to listen to him tell her that he was gay. Mandy proposed that they start officially dating each other, not only to hide Ian's homosexuality, but also to prevent Mandy from being constantly hit on by boys at school. They both happily agreed to this and became close friends as a result.
During Series 2, the dating ruse was taken even further when Mandy started having sex with Ian's older brother Lip in the boys' bedroom, in order to further make people think that Mandy and Ian were a thriving, happy couple. The illusion was shattered however when Mandy became pregnant with Lip's child and was unable to abort it due to her family's strong Catholic beliefs. Her family invited her supposed boyfriend, Ian to their house once they discovered the pregnancy, and while Ian feared for his safety, they actually congratulated him. Unfortunately, they also told him he would have to marry Mandy now, and being so desperate to keep his secret Ian agreed, leading to plans for a wedding. Lip saw what his brother was doing and tried to reason with him, but Ian resisted, forcing Lip to seek out the Maguires and admit he was actually the father. The Maguires were less than pleased with this, and Paddy warned him to keep away. The two separated when Lip had an affair at college, leading Mandy to the psychotic Joe, who is abusive. They break up when Jaime finds out Joe is beating Mandy and puts him in the hospital, Lip and Mandy got back together and were supposed to be attending their engagement party the night she dies.
Mandy went into labor in Series 3, after a mugger grabbed Sue Garland's bag and hit Mandy over in the process. She gave birth to a daughter who she named Katie, after her mother whose real name is Katherine. Lip tried his best to visit Mandy and Katie in hospital as well as to see his daughter afterward, but the Maguire family stopped him every time. Later, Lip and Mandy realized that they had feelings for each other and admitting they had done everything in the wrong order, decided to give a relationship a try. The couple got on well and though Mandy's fiery temper and Lip's occasional irresponsibility would get in the way, the couple's main problem was Mandy's interfering family. Though the Maguires were now unable to stop Lip from seeing his daughter, they continued to cause problems by giving Mandy money for Katie, which Lip saw as crime raising his daughter. Paddy tried to put the differences behind them by giving Lip the gift of brass knuckles and a job in the family business but Lip was disgusted at the violence involved and demanded Mandy stop taking money from her family and let him pay for his child.
It later caused more friction when Lip discovered Mandy had still been receiving money from her family, as he naively thought his small contributions had been enough. The couple eventually split when Lip moved away for University and the couple tried an open relationship, on the condition they didn't get emotionally involved with anyone else. When Lip came home for an end of term break he was followed by a girlfriend from University and though his family was shocked, Mandy admitted they had an open relationship, though Mandy hadn't been involved with anyone else. They got back together until her death. They were planning to attend their wedding.
In the final episode of Series 6, Mandy was lured away by Maureen, an emotionally unstable woman who lost her daughter to heroin and blamed Paddy Maguire for her death. Mandy realised that Maureen was the person who kidnapped Paddy too late, and was lulled into a trap. As Maureen disappeared to put the kettle on, Mandy followed to see what she was doing, and to her horror, found a broken gas pipe which Maureen had sabotaged. Maureen then returned to the kitchen with matches. Mandy desperately began fighting to get the matches from Maureen just as Paddy and Micky turned up outside to save her, however they could not gain access into the house. Inside the house, Mandy had fought her way into the hallway and ran toward the door. Just as Micky pressed the doorbell, creating a spark, Maureen lit a match. The gas then ignited, causing the house to explode, killing both Maureen and Mandy and throwing Micky across the street. The episode then cut to Kelly, reading a book to Katie whilst trying to hold back tears. Meanwhile at The Jockey, Mandy's funeral reception was taking place. Micky, who survived the blast, admitted that he felt responsible for Mandy's death.
Mandy's wake was held at The Jockey, where Jamie took the stand and sang an emotionally-charged Irish ballad. Micky revealed that Paddy and Mimi were going to take Mandy's ashes to Ireland to scatter them over the River Shannon. The crowd then started Irish dancing in celebration of Mandy's life, but it soon turned into a riot, topped off by a drunken Frank taking Mandy's ashes and throwing them in the air, across the Jockey - the episode closed on a freeze frame of an enraged Paddy lunging towards an oblivious Frank.
Mandy shares the same headstone as her older brother Fergal, who was murdered in retribution for Jamie's murder of a rival crime family's son. Paddy sadly visited Mandy's grave in Series 7 when his new grandson Connor was christened.
In Series 8, Mandy returned as an apparition to give her stressed mother some advice. The first time Mimi saw her daughter, she appeared pregnant and asked if there was "room for a little one" at the table where Mimi sat. Mandy later appeared again to Mimi in a hospital Chapel where Mimi was praying after Cilla and Shane were taken ill and had a full conversation with her to convince her of her recent mistakes. Mandy also mentioned Joe Pritchard and when Mimi angrily asked how she could still be "sweet on him", Mandy replied that she was still "kind of seeing him". This has been the only allusion to Joe being dead.
- Samantha Siddall who plays Mandy and Rebecca Atkinson who plays her on screen sister in-law Karen are close friends in real life.
- Mandy's gravestone, shown in Episode 702, states that Mandy was born in 1986. However, she appeared to be in the same high school year-group as Ian Gallagher (born in 1989) when she is first seen in Episode 103.
- After giving birth, Mandy had disproportionate breast enhancement surgery.