- "Sure'n I hope you're considerin' the future, Mr. Eastwood?"
- —Maggie McFly to Marty at the Town Festival.
Maggie was a no-nonsense woman, not prepared to take any hardships from anyone. Despite this, she loved her husband and their son William dearly and was caring, ready to help those in need.
Seamus, Maggie and Seamus' brother Martin left Ballybowhill, Ireland in the early 1880s with hopes of a better life. They gradually headed west, eventually ending up in Hill Valley, California, although it is unknown exactly when they arrived there.
They either lived in, or passed through, Virginia City, Nevada before arriving in Hill Valley. It was there that Martin McFly lost his life - whilst trying to prove that he wasn't a chicken, he was stabbed in a saloon with a bowie knife. This had an effect on Seamus as well, who remembers his brother and tries not to make the same mistake. Seamus encourages others to learn from Martin's mistake and not let what other people think of them take over their lives.
After settling into Hill Valley, they lived and worked in a farmhouse 14 miles outside of town, together with their son William who was born in April 1885, "the first McFly to be born in America". Since William was a few months old by September 1885, and the townspeople knew the McFlys quite well, they may have been living in Hill Valley for a couple of years by 1885. The couple worked hard on their farm, often going into town - where local outlaw Buford "Mad Dog" Tannen aside, they were liked by all the townspeople.
Seamus and Maggie first met Marty McFly on September 2, 1885, who had time traveled back from 1955 (and originally from 1985) when he crashed into their fence whilst running from a bear. Seamus found Marty unconscious and he and Maggie took him in, putting him in their bedroom while he recovered.
After Marty woke up, he discovered who his great-great grandparents were. As he couldn't tell them his real name, he called himself "Clint Eastwood". Seamus in particular had a feeling about Marty, that they should look after him - especially after seeing Marty hold William, who normally hated strangers. Seamus and Maggie let "Clint" stay in their barn overnight, after telling him about his friend Doc Brown being the local blacksmith in town. The next morning, Seamus gave "Clint" a lift to the railroad tracks into Hill Valley.
Maggie and Seamus went into town themselves two days later for the Hill Valley Festival, which was celebrating the arrival of the new courthouse clock. There, they met "Clint" again (who had found Doc by this stage). Buford Tannen showed up and after trying to shoot Doc, challenged Marty to a duel by calling him chicken. Seamus and Maggie overheard this and afterwards told Marty the story of his namesake Martin in Virginia City, suggesting to Marty that he start thinking about his future.
Maggie could perhaps have been a (great-)aunt to either Sam Baines or Stella Baines, thus explaining her similarity to Lorraine. If so, her family was connected to the McFly family at least twice, with her marrying Seamus, and Lorraine marrying their great-grandson. This would make Lorraine and George second cousins.(non-canon speculation)
Behind the scenes
Maggie looked like Lorraine Baines McFly, due to actress Lea Thompson playing both roles; this serves the running gag of Marty being knocked unconscious, taken care by a female character played by Thompson, and then waking up scared realizing he is in the "wrong" timeline. Other than that, it is not established that the two characters played by Thompson were directly related.
- Harold McFly (medieval ancestor-in-law)
- Jennivere McFly (medieval ancestor-in-law)
- Martin McFly (brother-in-law)
- Pee Wee McFly (nephew-in-law)
- Seamus McFly (husband)
- William McFly (son)
- Arthur McFly (grandson)
- Sylvia McFly (granddaughter-in-law)
- George McFly (great-grandson)
- Lorraine Baines McFly (great-granddaughter-in-law)
- Dave McFly (2nd-great-grandson)
- Linda McFly (2nd-great-granddaughter)
- Marty McFly (2nd-great-grandson)
- Jennifer Parker (2nd-great-granddaughter-in-law)
- Marlene McFly (3rd-great-granddaughter)
- Marty McFly Jr. (3rd-great-grandson)
- Marta McFly (5th-great-granddaughter)
| The McFly Family|
Marty McFly | George McFly | Lorraine Baines McFly | Jennifer Parker | Seamus McFly | Maggie McFly
Dave McFly | Linda McFly | Martin McFly, Jr. | Marlene McFly | Arthur McFly
| The Brown Family|
Dr. Emmett "Doc" Brown | Clara Clayton Brown | Jules Brown | Verne Brown | Einstein
| The Tannen Family|
Biff Tannen | Buford Tannen | Griff Tannen | Irving "Kid" Tannen
| The Strickland Family|
James Strickland | Roger Strickland | Irene Strickland | Edna Strickland | Gerald Strickland
Match, Skinhead & 3-D | Goldie Wilson | Douglas J. Needles