Maggie is first introduced in the penultimate episode of the tenth season. Cristina Yang interviews various different candidates to replace her as the head of her department in Cardiothoracic surgery. Maggie, having finished high school and medical school early, becomes head of her department at 31 years old. It is revealed in the season 10 finale "Fear (of the Unknown)", Maggie's biological mother was Ellis Grey. Richard Webber is shocked by this, knowing he is her biological father. Throughout the eleventh season, Webber struggles to deal with the revelation and keeps it from Maggie. Maggie explains that 2 years prior, she looked for her birth mom and needed to go to the court for the records. She found out that she was dead, but got her name, and discovered that she was an amazing surgeon that worked at the Seattle Grace Hospital. She stated that that's not why she took the job, but it's "a little bit" why she took the interview.
When Maggie identifies Meredith Grey, her half-sister, she attempts to bond with her. However, Meredith attempts to take charge and conflicts with her on a case. After several arguments, Maggie tells Meredith that she is her half-sister. Meredith, in disbelief, points out that she would know that her mother would be pregnant when she was five years old, as Maggie is five years younger than her. Meredith works with Alex Karev to decide whether Maggie is lying or if she is wrong. They discover she is correct. Meredith gets flashes back to the time of her mother's crisis at five years old, and remembers being at a hospital and hearing a screaming baby. After several weeks, Webber admits to Maggie that he is her father. Maggie is angry at him. Later, when he comes to apologize for not being there as a father, Maggie corrects him and says he is not her father, as she has adoptive parents she loves, and that she is only angry because of his dishonesty and making her look foolish and naive for weeks. Derek and Meredith decide to have Maggie come over to their house for dinner, to get to know Maggie better. After Maggie says yes, Derek decides it is a good idea to ask Richard as well. Richard reluctantly agrees. The night of the dinner, Maggie and Richard show up, but no-one answers. Maggie leaves him after that. Maggie develops a relationship with radiologist, Ethan Boyd. She admits her awkwardness in relationships, having ended her previous engagement to Dean, who she was unable to sleep in the same bed with. Meredith ends up in a bind with no one to watch her kids, so Maggie offers to watch them. Richard and Maggie eventually come to good terms, and Maggie's helping with Meredith's kids help her integrate into her family home more. Maggie learns about the histories of Meredith, Alex and Callie Torres as they commemorate their dead friend, George O'Malley.
After Derek's death, Meredith abandons Seattle for a year, leaving Maggie a note. Maggie is shocked by this, and admits she felt abandoned by Meredith right when they were starting to become a family. She invites Webber to a Christmas party, noticing his loneliness. Meredith eventually returns home, hugging Maggie and re-integrating into her old family home. While Maggie and her colleagues are busy dealing with Keith Gardner after a tunnel collapse, Maggie gets a phone call that her parents are getting a divorce. Her mother confesses to her that she'd been having an affair with their car serviceman for eleven years and they had only stayed together for her sake, but were getting a divorce now that she was out on her own. Maggie is emotional about this, but she doesn't want to bother her colleagues about it. Later, she confides in Meredith over the issue, but feels it is silly in comparison to her husband's death. Meredith, however, says she can tell her anything and she should come to her whenever she is having a problem. After Meredith buys her house back from Alex, she invites Maggie and Amelia Shepherd to move in with her. They assist Meredith in caring for her kids. Maggie also often mediates between Meredith and Amelia, who frequently get into fights over little things. She often helps with Meredith's three kids, with each "sister" in charge of getting one kid ready for the day.
In the twelfth season, Maggie develops a relationship with intern Andrew DeLuca. Maggie sleeps with intern Andrew DeLuca after a short conversation and a couple of drinks at Joe's. They agree it is a one-time thing, before they have sex again. Maggie and DeLuca keep their relationship private to avoid difficulties at work. DeLuca grows frustrated with their secrecy, so Maggie abruptly decides to go public. When this happens, DeLuca becomes uncomfortable with the way he is viewed by others because he is dating an attending, and doesn't want others to think he is getting special treatment. He also feels intimated by her authority. When she confronts him, he ends things with her. Maggie attempts to move on and develops an interest in Nathan Riggs (Martin Henderson). However, she is unaware Meredith has a sexual relationship with him. She confides in Meredith about this at Amelia's wedding to Owen Hunt. In the thirteenth season, Maggie is personally hurt by Meredith lying to her about Alex's actions assaulting DeLuca. She tells her to never lie to her again, but she does so immediately afterwards about Riggs. Riggs tells Meredith she needs to tell Maggie. Maggie works up the courage to ask Riggs out, but he rejects her.
