In the film, a depressed Lady reveals to Jock and Trusty that Jim Dear and Darling have been acting strange lately. Firstly, Jim Dear wouldn't play with her when he came home the other day. He instead asked Darling if she was all right because he was worried about her being home with only Lady, whom he referred to as "that dog", much to her shock. Next, Darling wouldn't take her for a walk the day before and when she tried to play with Darling's ball of yarn Darling struck her on the back.
Upon realizing why Jim Dear and Darling are acting so strangely, Jock and Trusty assure Lady that she has done nothing wrong, and explain that Jim Dear and Darling are expecting a baby, which they say are very cuddly and adorable miniature humans. Then Tramp stops by and tells him that with one coming, things will start to change for the worse for Lady. Jock insists that he is wrong, and shoos him away, but before he leaves, he tells Lady that when the baby moves in the dog moves out.
Over the next few months, Jim Dear and Darling prepare for the baby such as throwing a baby shower and Darling sends Jim out to get her some food such as watermelon and chop suey. Then on a stormy night in April, Jim Dear calls his Aunt Sarah to let her know that the baby was born and that it's a boy. Although it is never mentioned what they named him, it is likely that they named him Jim Jr. after his father. One day Lady hears Darling singing him to sleep and finally learns what a baby is when she goes into his room with Jim Dear to get a closer look and instantly decides that he can't be a bad thing like Tramp predicted.
A few days later, Jim Dear and Darling are leaving for a vacation. Lady thinks that they are abandoning Jim Jr., but Jim Dear assures her that they will be back soon and that she will be a great help to Aunt Sarah in taking care of Jim Jr. When she arrives, she runs to his room and begins to coo at him, but when Lady comes in, she shoos her away and Jim Jr. starts crying, which Aunt Sarah believes is because Lady scared him, but it is more likely that he is afraid of her shouting. She then sings to him, which sounds horrible compared to Darling.
Later on in the film when Lady is chained to her doghouse as punishment for running away, she sees a rat sneaking into Jim Jr.'s room and tries to warn Aunt Sarah but she won't listen and orders her to be quiet. Hearing her frantic barking, Tramp comes and she tells him about the rat. He rushes into the house and into Jim Jr.'s room and fights it, but accidentally knocks over Jim Jr.'s crib in the process, making him cry. Tramp is victorious, and Lady, having managed to break free from the chain, comes in and checks to see if Jim Jr. is unharmed. Though he is (and stops crying among seeing Lady checking on him), Aunt Sarah comes in and thinks that Tramp attacked him (and Jim Jr. starts crying again due to Aunt Sarah barging in and scooping him up). She shoos him into a closet and calls the dogcatcher to come and take him to the pound. Lady tries to show her the rat but she grabs her and locks her in the cellar. After the dogcatcher comes and takes Tramp away, Jim Dear and Darling come home and talk to Aunt Sarah about what happened. Jim Dear releases Lady from the cellar, believing that there's been a mistake and she shows him, Darling, and Aunt Sarah the dead rat in Jim Jr.'s room, proving Tramp's innocence and Jim Dear and Lady go off to rescue him.
On Christmas Eve, Jim Jr., now one years old, is seen having his picture taken with Lady, Tramp, and their four puppies, Scamp, Annette, Collette, and Danielle. Scamp starts playing with Jim Jr.'s pajamas, but Darling picks him up and goes upstairs to his room for a nap.
Jim Jr., now two years old, is first seen playing with Scamp, which ends with him making a mess while trying to catch a ball. When he goes with Angel to see his family after he ran away, Jim Jr. is shown missing him and is given a family photo by Jim Dear. At the Fourth of July picnic, he is overjoyed to see him "return" but is surprised to see him run away again. He is last seen welcoming him and Angel into the family.
Jim Jr. is portrayed as a girl named Lulu in the 2019 Disney+ live-action remake.
- His appearance closely resembles that of Michael Darling from Peter Pan, due to his pink pajamas and blue eyes which makes them almost very identical, but Michael is slightly older (about four years old) than him (who in contrast is only a baby).