|Franchise||Back to the Future|
|Vehicles and/or Gadgets||None|
|Occupation||Marty McFly's great great grandfather and Clint Eastwood's friend|
|Residence||Hill Valley (1885)|
Seamus, Maggie and Seamus's 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 remembered his brother and tried not to make the same mistake. Seamus encouraged others to learn from Martin's mistake and to 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.
Tannen took a dislike to Seamus for some unknown reason and told Seamus not to step into the Palace Saloon. Seamus, however, did not appear to take much notice of Buford, and the two men never actually encountered each other.
When Batman, Wyldstyle and Gandalf arrived at Hill Valley they found him in stuck in a pile of manure, however, he was later rescued by the trio using a crane summoned by the Locate Keystone. Sometime later he was angrily kicked by Mad Dog Tannen out of the Palace Saloon because he called him by his alias. After he and other people saw Vortech's power, they fled in fear.
- He is voiced by Michael J. Fox, who also played Seamus in Back to the Future: Part 3.