Mary grew up with her older brother, Steve, and her parents, Detective John McGarrett and housewife/CIA agent Doris McGarrett in Honolulu, Hawaii.
Twenty years ago, Doris died, leaving their father to raise them alone. Unknown to Mary and Steve, John was investigating the Yakuza that ultimately led to Doris's "death".
Fearing for Mary and Steve's safety, John sent them to the mainland, separated, with both of them not knowing his true intentions.
Dori's was presumed to have a huge emotion toll on Mary and her relationship with John and Steve became strained because of it. Sometime after that, she moved to Los Angeles, and returned to Hawaii. After getting kidnapped, Mary returned to Los Angeles.
Although Mary is an adult, her behavior is often similar to a teenager. Steve feels responsible for her, as Mary is often out of control and erratic. Mary also seems to resent her father, and her brother in a similar way, because they see each other. Mary believes that she and Steve meant nothing to their father, and that their father didn't love them. Despite her tough exterior, Mary is shown to deeply love and care for her brother, even though they haven't talked to each other in ten years. Upon her departure, Mary hugged Steve goodbye, signaling that she forgave him for the ten year gap.
Upon arriving in Hawaii, Mary is arrested for a federal offense, as she signaled a smoke alarm. Her brother, Steve McGarrett, releases her under his custody. As the two officers leave, Mary apologizes for "messing up." Steve however comments that he didn't see Mary at their father's funeral. Mary then reminds Steve that she hasn't seen him since their mother's funeral, and ponders out loud why they can't have reunions like normal families have. During a deleted scene you can see that Mary was at the funeral but turned up high. They only used part of that scene when Steve is reflecting on the funeral and you can see the back of Mary's head. Later on a crime scene, Steve tells Mary to stay in the car. Mary tells him that she is not a dog and promptly starts barking. At Steve's request, Mary is taken to his office for the time being.
A couple of hours later, Steve apologizes to Mary for the wait. Mary is apparently not forgiving, as she asks if his idea of a welcome plan was to pin her to a cop; Steve replies that he's trying to keep her out of prison. Mary then stands up and fixes the collar of his shirt, saying that "you're like seven years old." Mary then confesses that she felt that she was invisible to their father. Steve explains to Mary that their father did love them; he just didn't know how to show it. Steve introduces Mary to Detective Sergeant Danny Williams (2010), who, like Steve, makes fun of Danno's tie, and Kamekona, who is her "babysitter," much to Mary displeasure. Shortly after Steve and Five-0 return, Kamekona confesses that Mary slipped out. Steve is able to find Mary by their father's graveside, and walks over to her carrying a paper sack. Mary sees him and asks what's in it. Steve smiles and tells her lunch. They both sit down and have the family picnic that Mary wanted.
A week later, Mary is living in Steve's house. Early in the morning, Mary bumps into Lieutenant Catherine Rollins, who was staying over with Steve. Mary introduces herself and comments on the "big night" Catherine had last night. Catherine looks confused, and Mary tells her that the house is old and has thin walls. Catherine, realizing what she means, blushes. Mary reassures her that it isn't a problem. "I'm planning to have a lot of guys over, so it'll be a fair fight." Later that night and with Catherine at work, Mary is sitting down watching TV when a commercial advertising the "Hawaii Chair" comes up. Mary tries to change the channel, but is unsuccessful. Mary tries to find batteries, also unsuccessful, and pulls out the "Champ" box. She calls Steve, and asks him why there are pictures of burnt people and postcards from Japan in the toolbox. Steve instructs Mary to put the box back, as it is evidence for a crime. Although Mary tells Steve that she will put the box back, Mary is seen searching in the box and taking pictures.
In early January in the new year, the Champ box is stolen. Steve realizes that the only person that he talked to about the box (in Nalowale (episode)) was Mary, and hurriedly goes to check on her with Danny. They find no sign of Mary in her rental home, but they find blood and a tooth, which, later proves to not be Mary's. Mary calls Steve on her phone. She explains she is in a trunk in a car, and Steve tells her to kick the back taillight and tell him what she sees. With Mary's description, Steve and Danny are able to rescue her. Steve brings something for a bandaged Mary to drink in his office. Mary explains to Steve that she told everyone in HPD about the case of their mother's murder, and also tells him that since the Champ box is stolen, she has another way: her cellphone. Mary took every picture in the box.
Later that day, Steve takes Mary to the airport to L.A. They share a hug as the airport calls for the passengers to board. Steve promises Mary that they will see each other again.
Mary returns (with a much shorter haircut) in Kalele, having acquired a job as a flight attendant.
Mary reappears in Ohuna, now working as a caregiver, and comes to Hawaii witha senior citizen Morty, who has always wanted to visit Hawaii.
When Steve tries to arrange the reunion between Mary and Doris, she is very reluctant for it. Doris later sees her at John's grave, crying. Mary is angry at Doris for faking her death.
Later, Mary and Morty are seen on the beach. Morty tells Mary that he himself once had a fight with his daughter, then they didn't speak for years, before Morty received news that his daughter was dead. He convinces Mary to give Doris a second chance, which she does.
Mary returns in Ua Nalohia (episode) where it's later revealed that she's adopted a baby girl whom she names Joan McGarrett . After Mary falls ill, Steve is forced to bring the baby to the office while Mary recovers. During that time, Steve, Chin and Danny take turns in caring for Joan.
After Mary recovers, she returns to the building to get Joan and learns that Steve actually enjoyed his time with the baby.
- It is implied that the creators of Five-0 named her after Mary MacArthur, the older half-sister of James MacArthur, who played Danny in original 1968 Hawaii Five-O. Mary MacArthur was an inspring stage actress, but died from polio at the age of 19 in 1949.