- “Rather inclined to giggle. Doesn't put things away.”
- ―Jane's description on Mary Poppins' tape measure
On one hand, Jane can be kind, friendly, and caring. On the other hand, it is implied that she, along with Michael, can be very mischievous. By the time of the film's opening, they had gone through six nannies in the past four months. They try to please George (whom they address as simply "Father" throughout the film) but have often failed, leading to some insecurity.
At the beginning of the film, the current nanny, Katie Nanna, had quit due to the children's rampant misbehavior, who had gone missing, having run away from her for the fourth time that week. Later, Constable Jones brings them home, revealing that they had not run away, but had merely lost track of her while having trouble with their kite. Jane notes that it wasn't very good, as they had made it themselves. They are then sent upstairs.
Later, Jane and Michael approach George with an advertisement for a new nanny. It calls for a fun, caring one. It also hints at the pranks they had played on former ones. However, he dismisses it as ridiculous, tears it up, and throws the pieces in the fireplace. The next morning, Jane and Michael are watching the line of nanny applicants, whom appear to be the exact opposite of the kind they wanted. However, they watch in amazement as a sudden wind blows them all away. Then, a person who fits their advertisement comes to their home.
Later, the woman meets Jane and Michael and amazes them with her bottomless carpet bag. She then measures them with her tape measure. Instead of numbers, it reveals something about the person. For Michael, it says he is "extremely stubborn and suspicious." For Jane, it says that she is "rather inclined to giggle and doesn't put things away." She then allows herself to be measured, and reveals her name is Mary Poppins, which Jane notes as being lovely.
Next, Mary plays a game with them, which turns out to be tidying up the nursery with the use of magic. Jane is able to snap her fingers and toys put away themselves. Afterwards, she and Michael accompany Mary on an outing in the park. On the way, they meet her friend, Bert. He laughs when they say they are going to the park. He is able to trick Mary into transporting them into one of his pictures, which depicts the English countryside. Jane and Michael play at a fair for a time, while Mary and Bert take a stroll.
Later, they meet at Mary's private carousel. Jane is amazed at the idea of that and is even more so when the horses jump off. She accompanies the rest of the group on a ride. She later witnesses Mary effortlessly win a horse race and learns about her special word, Supercalifragilisticexpialidocious. However, the outing is cut short when it begins to rain, which washes the drawing off. Back home, Jane is forced to take medicine to prevent a cold and is surprised when it changes flavor and becomes a lime cordial for her. She and Michael try to discuss the day's events with Mary. When they are about to fall asleep, she sternly denies it.
The next day, Jane and Michael accompany Mary to see about her Uncle Albert, who has floated into the air due to too much laughing. Bert is there as well and is eventually stricken with the "disease" as are Jane and Michael. They have a good time, even having tea while floating thanks to Mary's magic. However, everyone sinks to the ground due to sadness at having to leave to go home. Later, Jane and Michael tell George of their adventures, which worries and angers him. He sends them up to the nursery as usual, and then decides to try to fire Mary, but is prevented by her planning an idea into his head to take them on an outing to the bank he works at.
The next morning, Jane and Michael go with George to the bank as "proposed". They are confused and frightened by the descriptions of finance, and simply want to use their money to feed birds at St. Paul's Cathedral. When Michael's is taken by the owner, Mr. Dawes Sr., he and Jane unknowingly cause chaos. Frightened, they escape and eventually run into Bert. They tell him that they believe George hates them. He helps them to understand that George is simply trapped in a cage due to his responsibilities and has no one to turn to but himself, hence his strict and uncaring attitude toward his family.
At home, Bert is employed by Winifred to watch Jane and Michael as it is Mary's day off. They assist him in cleaning the chimney but get drawn up it, just as Mary warns them of the danger. She and Bert come after them and decide to lead an exploration of the rooftops. They meet Bert's chimney sweep friends and have a party, which ends up in the Banks' living room. After Bert and the chimney sweeps leave, it is revealed that George is due to be fired because of the events at the bank. To try to fix things, Jane and Michael give him the tuppence that caused the incident.
The next morning, Jane and Michael are saddened as Mary is preparing to leave. They are called by George, though they think it's not him initially due to the voice being so happy. Going downstairs, they find that he has had a change of heart, and has fixed their broken kite. He takes his family kite flying, symbolizing his intent to become a more involved father and husband.
In 1935, Jane is now an adult who works for a women's labor organizer, and the film takes place during the time of the Great Slump. Unfortunately, she is not very strict and harsh with them in as much the same way that George treated her when both were kids, likely as a result of Michael's depression over his wife's death.
In the opening, there is a chaotic scene where the kids, who have been taught by Jane to take care of themselves, hear reports from some lawyers from the bank, that their house is going to be repossessed to repay a large loan that Michael took from the bank to cover his losses during the Great Slump. The bank is now run by Mr. Dawes Jr.'s nephew, William Weatherall Wilkins who is a corrupt businessman using the advantage of the Great Slump to repossess houses for no reason, though Jane is unaware of his plot. By this point in their lives, Michael and Jane have also stopped believing in Mary's magic, dismissing it as their imaginations.
The only way to avoid having their house repossessed is to repay their loans by Friday at midnight and the way to do that is to locate the shares in the bank that George left them. Michael and Jane search the attic and the downstairs' office for the shares, but they come up fruitless. At the same time, Mary returns to the household to restore order for the Banks family, and, just as in the original film, hires herself as the kids' nanny despite the father's skepticism.
Michael and Jane try to find records of George's shares at the bank with Mr. Wilkins at the bank, but he pretends to not have any on file and, as soon as their backs are turned, he throws the records in the fire to destroy any evidence of it.
One day, the kids embarrass Michael during a visit to the bank and tell him that the rumors about the bank repossessing the house are true. Michael then mentions Kate, asking if their family haven't lost enough already and Jane feels concerned that her brother is now nearing tears. The Banks children cheer him up by singing a song that Mary Poppins sung to them, and Michael feels better, telling Jane that even if the bank repossesses their house, they still have each other.
The next day, the Banks family packs up to evacuate the house, knowing they have lost. But, when Georgie brings the repaired kite from their first time with Mary, Jane realizes that they repaired it using a drawing on the back of the shares' certificate. Realizing that they now have proof, Michael and Jane rush to the bank in order to save their house. With help from Mary and her new partner, Jack, the kids go to "turn back time" - climbing Big Ben in order to alter it so that it doesn't strike midnight before Jane and Michael can prove their shares.
Jane and Michael try to enter the bank, but Mr. Wilkins has his henchmen, Hamilton and Frye, lock the door. So they use the kite to bring the certificate to Mr. Wilkins' office and, after one of the henchmen, who sympathizes with the Banks family, changes sides and allows them inside, they go up to Mr. Wilkins' office. Unfortunately, they are missing the piece of the certificate with George's signatures on it, and Wilkins seemingly has won. An angered Michael tells him he can have the house, that he should have listened to the kids earlier, and that they have all they need. However, lucky for them, Mr. Dawes Jr. comes into the room out of nowhere to save the day; having been tipped off by Mary of Mr. Wilkins' true intents, he chastises him for his corrupt practices, and fires him from his position as president of the Fidelity Fiduciary Bank, taking back his job.
Mr. Wilkins, before leaving, tells Mr. Dawes Jr., in defense of his practices, that he's made the bank more money than the Dawes ever did, but Mr. Dawes Jr. scoffs at his defense and then orders Hamilton and Frye to escort him away while allowing the Banks family to use George's invested tuppence from the first film, and the return from the investment, to repay their loan in full, saving the house in the process. He also allows them to keep their bank shares without having to spend them, thus guaranteeing the kids, their future as adults.
The Banks family celebrate their victory by flying balloons at a Fun Fair, and all realize that Mary's magic has always been real. Eventually, both realize that the time has come for Mary Poppins to leave and thank her for saving their house.
- An adult Jane Banks would be the narrator for an edition of the Read-Along version, as that role would be reprised by an adult Karen Dotrice.
- In the Mary Poppins novels, Jane and Michael have twin siblings, named John and Barbara, and a baby sister, Annabelle, who is born in the second novel.