- "Arthur McFly was outside waxing the car. He waved his rag as the two boys approached. / "Go and talk to him right away," Marty urged. / "About what?" / "About college." / "I'll get to that. I've gotta introduce you first." / "No," Marty said, stopping at the edge of the sidewalk. "I'm not moving until you talk to him about college." / "Sure..." George said hesitantly. / He walked over to his father, looked back over his shoulder at Marty. In order to make him feel more secure, Marty meandered toward the porch of the house so that he was out of George's line of vision. He was actually closer around the side of the house, and could hear the conversation quite clearly. / "Who's your friend?" Arthur McFly asked. / "A new guy from school," George replied. "Listen, Dad, I have an important decision to make and, well, I really need some advice." / Only a C-plus beginning, Marty thought, although the bit about needing advice was probably good psychologically. / "Gee, son, I'm kinda busy here," George's father said. "Couldn't it wait a few days?" "
- —From Back to the Future by George Gipe (quote, pages 158 and 159)
When Arthur was 16, he enlisted in the army and was sent to France during World War I. When the military brass realized he lied about his age, he was discharged for fraudulent enlistment before he got the chance to see combat. Arthur took this hard and felt ashamed of it for years afterward. His confidence ruined, he never tried to do anything that would make him a success, and had a permanent fear of failure. Arthur did have skill with numbers, but his checkered service record from the Army made it tough for him to get a legitimate accounting job. When Prohibition went into effect in 1920 he started to work for Kid Tannen, the notorious organized crime boss in Hill Valley. Economic downturns forced Arthur to stay with the Tannen Crime Family, albeit reluctantly. Kid Tannen disliked Arthur, but was forced to keep him on due to no respectable accountant wanting to take him on as a client and the other members of the Tannen gang being so dumb they could not do basic math.
In 1955, when Marty convinced George to talk to his father about attending college, Arthur gave George a number of excuses and reasons not to attend, particularly due to his own inability to be anything. Though for Arthur's part he believed he was trying to protect George from the same shame he endured, despite what he had done was reckless and dangerous. Upon witnessing this Marty was outraged and had very little respect for his grandfather 
George referenced his disappointment in his father to Marty in 1986 right before Marty went back in time to 1931. Upon arriving in Depression-era Hill Valley, Marty walked into the Sisters of Mercy Soup Kitchen where he saw a wimpy Arthur McFly (a dead ringer for 1955 George in looks, voice, and personality) being bullied and harassed by Kid Tannen. After taking Artie’s signature white fedora with a red headband, Tannen ordered him back to the safe house before he can be served a subpoena.
Later, Marty found that in order to accomplish his mission he had to help 17-year-old Courthouse Clerk Emmett Brown serve a subpoena to Arthur McFly. It was established that Arthur was the accountant for Kid Tannen's illegal speakeasy business. Recovering the hat from Kid Tannen, who had been using it as a trophy and Peanut Bowl, Marty had Einstein track Arthur down. They discovered the Safe House but found Arthur wouldn't come down from the apartment without Kid Tannen's say-so. Marty eventually tricked him into coming downstairs using a 1980s tape recorder with Kid's voice, where he was then served the subpoena. Realizing that Kid would kill him for talking to the District Attorney, Artie began to panic but Marty and Emmett convinced him to go through with it. They gave him back his hat and he went to the courthouse to provide information on Kid Tannen.
Soon afterward, Marty and Doc are about to leave for 1986 when Marty suddenly begins to fade from existence, apparently rather painfully. A newspaper from tomorrow explaining that Arthur McFly was found brutally murdered on the courthouse steps, having been beaten and left for dead a few hours ago as revenge for answering the subpoena. Upon seeing his father’s image beginning to fade from a family picture, Marty realizes that the timeline is quickly catching up to his grandfather’s fatal wounds, preventing George's birth. With Doc’s help, Marty goes back in time several hours, avoiding his other self, and manages to save Arthur, rescuing him from Tannen's trio of thugs and urging him to lay low for a while. Arthur then asks Doc and Marty if they are "G-Men" (slang for United States special agents, which is short for Government Men). Deciding to just play along, Marty and Doc declare that they are G-Men and tell Artie to look out for a suspect named "Sylvia", which is the name of Marty's grandmother, Arthur's future wife.
Upon returning to 1986, Marty finds that history has been altered and in this new 1986 Kid Tannen was never sent to jail in 1931. The Tannen crime family now runs Hill Valley and endlessly harasses the McFlys. An old Kid Tannen then reveals that the only reason he didn't kill Arthur McFly was because his girlfriend made him promise not to. After escaping Kid and his goons, Marty and Doc then travel back to August 25, 1931, the night when Kid Tannen was supposed to be arrested. Doc explains that Kid Tannen was originally arrested when Trixie Trotter, his beautiful girlfriend and the singer at his Speakeasy, turned on him and gave Jennifer's grandfather, Officer Danny Parker, some incriminating evidence. But in this new timeline, this does not happen. Marty discovers this is because of two reasons: 1) Danny Parker has been traumatized by his seeing the DeLorean on the first trip to 1931 and 2) Without her smart friend and teacher Arthur McFly, Trixie is not confident enough to turn over the evidence. It turns out that while working for Kid, Arthur became fast friends with Trixie and taught her many things, including basic accounting. It is Felony Tax Evasion that Trixie has evidence of, but in this timeline she is hesitant and unsure of herself about presenting the evidence because Artie is not around for her to double check the numbers with. He had apparently left behind his pipe when he went into hiding, which she associated with intelligent men, and had kept it with her. She agreed to hand it over to Marty.
Using Arthur's pipe and Einstein's nose, Marty was able to track down Arthur at the theatre and convinced him to talk to Trixie. But their meeting was cut short by Kid Tannen, who immediately tried to shoot Artie in the head for answering the subpoena. Trixie begged for Artie’s life and made Kid promise to spare it. After that, she didn't feel right about betraying Kid and decided to destroy the evidence she had against him. Realizing that Trixie's feelings for Artie went much deeper than friendship, Marty decided to trick her into believing that Kid had gone back on his word and killed Arthur anyway in order to get her to give Danny Parker the evidence. Marty’s trick worked and Trixie turned over what she had on Kid to the police. Not long after, Kid Tannen was arrested.
Moments after the arrest, Trixie was overjoyed when she realized that Artie was still alive. They quickly began a romance. Marty was concerned about this because Trixie was not his grandmother, Sylvia was. But Doc noted that Marty wasn't fading into nonexistence and concluded that so long as Arthur did meet Sylvia and have baby George by 1938, it should be fine; He said to let Arthur sow his oats in the meantime. Unbeknownst to both of them at the time, Trixie was actually Sylvia.
In the months following the arrest of Kid Tannen and his gang, Arthur found that for the first time in his life, he was popular. For testifying against the gangsters, he quickly gained respectability throughout the whole of Hill Valley. He became so popular that people were practically beating down his door with job offers. As of October 12, 1931, Arthur worked at the Hill Valley Science Exposition, and due to interference by Marty was forced into firing Trixie from her role of Techne (due to her being a Canadian citizen). Later that night, he took her to Reno to marry her (5 years earlier than they were supposed to wed), thus making her a US Citizen (and allowing her to return to her role as Techne). While they were conversing with a concerned Marty, he asked Arthur about Sylvia Miskin, at which they revealed she was Sylvia. Arthur's father William McFly had been furious at them getting hitched, but had a change of heart upon meeting Trixie.
In 1986G, his son George referred to him in the past tense, implying that he had died by then in that timeline. His wife Sylvia was seemingly still alive in the original timeline in 1986, given that Marty referred to her in the present tense. He apparently had died in the original timeline when Marty was still a child. He maintained his pipe smoking habit for the rest of his life; Marty could barely remember Arthur, but did strongly associate him with the scent of his pipe.
- Back to the Future: The Game
Behind the scenes
Arthur might be visible in the photo of William McFly that Marty finds in the book A History of Hill Valley, 1850-1930. However, if Arthur was born in 1901 (making him 16 years of age when the U.S. entered World War I in 1917), William would then only have been 16 years old and a teen father at the time of son Arthur's birth, perhaps confirming the presence of a McFly curse upon the family.
However, it should be noted that Emmett Brown told Marty that Arthur graduated from Hill Valley High School five years before he did, suggesting that he was five years older than Doc. This would seem to suggest that he was born in 1909, making him 22 years old in 1931, but being only 5 when WWI started. However, if he was discharged from the military during World War I for being underage, it's likely he hadn't graduated by the time he enlisted. Since he was ashamed of being discharged, it's possible that he didn't return to complete high school until much later.
Notes and references
- Arthur was 16 when he enlisted; the American forces joined the war in 1917
- BTTF novel p. 158-160
- Back to the Future: The Game - Episode 2: Get Tannen!
- Back to the Future: The Game - Episode 5: OUTATIME
- In "OUTATIME", upon asking him to give Emmett more time to make it to his exhibit, Arthur refuses, prompting Marty to make a comment about how he refused to take him to the zoo at one time.
- Back to the Future: The Game - Episode 3: Citizen Brown
- Back to the Future: The Game - Episode 4: Double Visions