Jimmy's father died in the Great War, and his mother from Spanish flu. All he has left he tells Thomas Barrow are "cousins." He plays the piano, Anna Bates remarking at one point it was nice to have another piano player downstairs (In reference to William Mason who had also died).
Jimmy previously worked as footman to the Dowager Lady Anstruther. She wanted him to move with her to France, but he did not want to go, saying to Mr Carson he did not think he would fancy the food.
Jimmy started working at Downton alongside Sarah O'Brien's nephew Alfred Nugent. Jimmy has a "natural air of confidence and grace" that Alfred does not have, and "takes the role of first footman, as a natural right, a bone of contention." Mr Carson insisted he go by his proper name instead of his nickname upstairs. Jimmy was furious at that because he had never been called James in his life; even to Lady Anstruther he was called Jimmy. But Carson insisted he was James now and would remain James as long as he was at Downton.
O'Brien took advantage of Jimmy's ambition after Thomas bullied her nephew, Alfred. Whether she knew of Thomas' sexual orientation or simply suspected it, O'Brien urged Jimmy to stay close to Thomas due to his work experience, as Thomas was previously in the same job as Jimmy. O'Brien also said that because Thomas was at the time, Robert Crawley's valet, he had influence with him and Jimmy should stay on his good side. Since Thomas was attracted to Jimmy, Thomas gladly took him under his wing. Thomas at one point walked in on Jimmy when he was shirtless, which surprised him. Thomas also helped him with his duties, among other things teaching him how to take care of the households clocks, and stood very close as he did so - uncomfortably close for Jimmy. Jimmy felt uncomfortable with Thomas' behaviour towards him but, initially, O'Brien dispelled Jimmy's concerns and encouraged him to stay close to Thomas.
One night when Jimmy was asleep, Thomas sneaked in and kissed him thinking his feelings would be returned, but Jimmy was angry at Thomas for what he had done and Alfred caught him. Jimmy indicated he felt nothing for Thomas, leaving him devastated. After being encouraged to do so by O'Brien, Jimmy demanded that Thomas be sacked and given no reference and threatened to inform the police of Thomas's actions if Mr Carson did not do so. These actions were fuelled O'Brien's influence as Jimmy feared he would be suspected of being gay himself if he didn't act. His angry insistence on Thomas going alienated him from many other staff members, who could not understand what was going on. Mrs Hughes thought Jimmy might have unintentionally led Thomas on, calling Jimmy "a vain and silly flirt" and a "young whipper snapper." Jimmy was later persuaded to let the matter drop, and admitted that he personally never wanted things to go as far as they did. In order to placate Jimmy and to get him to agree to work with Thomas in future, Robert Crawley informed Jimmy that he had been made first footman, much to the surprise and dislike of Mr Carson. Jimmy was at that point, still upset that Thomas was staying on.
A year after the above events Jimmy still appeared uncomfortable around Thomas, not wanting to be seen to be alone with him or accept the gift of a drink when Thomas offered to buy everyone one. Whilst at Thirsk fair that a lot of the staff attended, Jimmy won some money betting on the Downton Abbey team in a tug of war match. At the last minute he asked Joss Tufton to join them - a large man - and Downton won and Jimmy won his bet. He proceeded to get drunk and flash his money around and whilst he was walking alone two men attempted to rob him. Thomas appeared and stepped in to protect Jimmy and was beaten and robbed in his place after Jimmy ran away and left him. Jimmy fetched Dr Clarkson and some of the other staff for Thomas and stayed silent whilst Thomas failed to say what had really happened and so avoided telling everyone that Jimmy left him alone to face the two men and get beaten.
Jimmy later went to see Thomas alone in his room and asked if he was alright. Jimmy said that Thomas was "brave" and that he felt bad for leaving him. Thomas revealed that he was following Jimmy that day as he had seen that Jimmy had had too much to drink. When Jimmy asked why he was doing this Thomas said that Jimmy "knows why", implying that he still had feelings for Jimmy. Jimmy said that he could never give Thomas what he wanted. Thomas replied that he understood and didn't ask for it, but would like them to be friends. Jimmy said he could do that if it was only being friends and the scene ended on them chatting over a newspaper.
By 1922, it seemed Ivy Stuart had captured Jimmy's attention. They danced, and were later kissing in the downstairs boot room when Alfred walked in on them. He did however upset her when he acted rudely toward Alfred about pursuing his dream as a chef, whereas everyone else downstairs was encouraging him.
Jimmy also helped Anna and Rose get away from a dance hall in York following a fight having broke out. He also sees Rose dressed as a servant after bidding farewell to Samuel Thawley. Rose asks him to be silent and she will be his friend forever.
Jimmy continues taking Ivy out, but when he heightens his advances beyond kissing, Ivy pushes him off, especially after he insists she return his good treatment of her.
His promised friendship with Thomas seems to have progressed, with them comfortably playing cards together. When Thomas goes to America as the temporary valet to Lord Grantham Thomas says he hopes to see Jimmy with a girl from the village by the time he returns. No signs of Thomas' former affection for Jimmy are outwardly seen.
Jimmy is ambitious and confident, being in Mr Carson's words, "sure of himself." His ambition leads him to be easily influenced by others, as he is manipulated by O'Brien to stay close to Thomas for the advantage it may give his job, when he is in fact unsure and uncomfortable around him. Jimmy is also a flirt, and could be seen as vain as he takes very good care of his appearance. He is called by Mrs Hughes "a vain and silly flirt". He flirts with Ivy and Mrs Hughes even thinks he may have unintentionally led Thomas on.
- The Military Service Act of 1916 states that everyone over 18 is valid to conscript; this means that Jimmy has to have been at least 18 in 1916 to be able to fight in the war.