James Edison Darmody is a fictional character in the television show Boardwalk Empire, played by Michael Pitt. He is one of the main characters in the first two seasons of the series. Unlike most of the other main characters in the series, Jimmy is not based on a historical figure, even though he may be inspired by Atlantic City politician and Atlantic City political boss Nucky Johnson's protégé, James H. "Jimmy" Boyd. Pitt is also the only actor besides Steve Buscemi to appear in every episode for which he is credited.

Jimmy is the son of Atlantic City political boss Commodore Louis Kaestner (Dabney Coleman) and showgirl Gillian Darmody (Gretchen Mol). His mother was only 13 at the time of his birth, while his father was 54. The Commodore's protégé, Enoch "Nucky" Thompson (Steve Buscemi), arranged for the Commodore to rape Gillian in return for being promoted to Sheriff of Atlantic City. Jimmy was raised by his mother with support from Nucky, who became a father figure to Jimmy; the Commodore was indifferent to his son. Nucky eventually took over the Commodore's political machine and set Jimmy up for great success when he grew up. Jimmy went to Princeton University, where he met his future wife, Angela (Aleksa Palladino). He left school to fight in World War I, leaving behind his pregnant girlfriend. In Season 2, it is revealed that he joined the Army because he felt guilty about a drunken sexual encounter with his mother and because he likely faced expulsion from college after assaulting a professor who had made a pass at her. Jimmy was stationed in France fighting on the Western Front against Germany. His leg was wounded by fragments from a German grenade, leaving him with a permanent limp.

In January 1920, Jimmy has recently moved back to Atlantic City after recovering from his war injury. He displays signs of post-traumatic stress disorder from memories of the trench warfare and refuses to go back to school. He ends up working for Nucky as his driver and bodyguard. Frustrated that he is not doing more, he and Al Capone (Stephen Graham) hijack a shipment of bootlegged liquor that Nucky sold to Arnold Rothstein (Michael Stuhlbarg) and gun down the drivers. One of them survives long enough to identify Jimmy, but Nucky gets him out of trouble by blaming the hijacking on the deceased Hans Schroeder (Joseph Sikora) and sending Jimmy to Chicago. There, he works for Johnny Torrio (Greg Antonacci) alongside Capone. When Nucky faces trouble back home from Philadelphia gangsters hired by Rothstein, he takes Jimmy back into his operation.

While visiting a military hospital in Chicago, Jimmy meets Richard Harrow (Jack Huston), a disfigured marksman. They become close friends, with Richard becoming Jimmy's main ally in bootlegging. Jimmy tasks Richard with killing a gangster who had disfigured Pearl, a prostitute Jimmy was fond of, who become addicted to laudanum as a result of the attack and ultimately committed suicide while in Jimmy's care. Jimmy reconciles with his father after the Commodore survives a poisoning attempt. Jimmy is deeply hurt when he finds out that it was Nucky who arranged for the rape of his mother by his father, and plots with his father and Nucky's brother Eli (Shea Whigham) to take back control of Atlantic City from Nucky.

By the beginning of season two, Jimmy is married to Angela, with whom he raises their son, Tommy. The Commodore has the New Jersey District Attorney's office arrest Nucky for electoral fraud and recruits Nucky's political allies. He and Jimmy then work on controlling the illegal alcohol being made and sold in Atlantic City. When the Commodore suffers a stroke, Jimmy takes over his operations. Jimmy is put in a difficult position when his partners in the bootlegging business — Capone, Lucky Luciano (Vincent Piazza) and Meyer Lansky (Anatol Yusef) — pressure him to kill Nucky. Though hesitant, Jimmy agrees to order Nucky's assassination. Nucky is shot in the hand, but Clifford Lathorp kills the assassin before he can finish Nucky off. Jimmy is left wondering if he is doing the right thing.

Jimmy enters a deal with Philadelphia gangster Manny Horvitz (William Forsythe) to supply him alcohol. Jimmy is short on cash and demands a $5,000 advance. However, Nucky has Owen Sleater (Charlie Cox), a former IRA assassin, blow up the warehouse where Jimmy's liquor is stored. Jimmy finds himself with no alcohol and thus no way to pay his debts. He ends up buying medicinal alcohol from George Remus (Glenn Fleshler) and uses it to make whiskey.

Nucky resigns as Treasurer of Atlantic County, and offers his surrender to Jimmy. However, this is all a part of Nucky's power-play. He has Chalky White (Michael K. Williams), the leader of the black community in Atlantic City, to get all the black workers in the city to go on strike. The strike cripples the tourist economy that the city runs on. With help from Owen, Nucky then buys whiskey from Ireland and starts selling it in Atlantic City. Nucky's bootlegging operation flourishes, while Jimmy and his partners are left with an inferior product and nobody to sell it to.

Meanwhile, Jimmy grows tired of Horvitz asking him to repay his debts, and contacts Horvitz's rival Waxey Gordon (Nick Sandow) to have him killed. A few days later, Waxey's henchman attempts to kill Horvitz in his shop, and manages to wound him, but Horvitz fights back and kills the assailant by sinking a meat cleaver into his head. Figuring out Jimmy is responsible from a matchbook in the dead man's pocket, Horvitz plots his revenge. Torturing Mickey for an address, Horvitz breaks into Jimmy's house, expecting to find him there. Since Jimmy is out of town, he instead finds Angela with her lesbian lover and shoots them both. Jimmy is devastated, and dulls the pain with alcohol and heroin. He attacks his mother when she talks about Angela's death nonchalantly. When the Commodore comes to Gillian's defense, Jimmy kills his own father, partially at Gillian's urging.