Street-performer Bert leads the audience to number 17 Cherry Tree Lane, where Mrs Winifred Banks returns home after a rally. She learns from the cook and maid that the babysitter, Katie Nanna, has left their service after their children, Jane and Michael, had run away "for the fourth time this week". She then has to tell her husband, Mr George Banks, who is a strict and driven bank worker. The children are returned home shortly after by Constable Jones, who reveals they were chasing after a lost kite. The children ask their father to help them build a better kite, but he dismisses them. Mr. Banks then places an advert in The Times for a stern, no-nonsense nanny but Jane and Michael present their own advertisement for a kinder, sweeter nanny. Mr. Banks rips up their letter and throws it away.

The next day, a number of sour-faced nannies are lined up outside the Banks' home, but then a young magical nanny named Mary Poppins descends from the sky with her umbrella. She produces the children's restored advertisement and agrees with its requests but promises the astonished banker that she will be firm with his children. As Mr. Banks puzzles over the advertisement's return, Mary hires herself, and she convinces him it was originally his idea. She meets the children and helps them magically tidy their nursery by snapping her fingers, before heading out for a walk in the park.

Outside, they meet Mary’s friend Bert again, a jack of all trades working as a street painter. Mary uses her magic to transport the group into one of his drawings. There are magical carousel ponies, funny little penguins, a fox hunt and a horse race which Mary wins. Describing her victory, Mary uses a nonsense word - Supercalifragilisticexpialidocious. A thunderstorm dissolves Bert's drawings and they find themselves back in the park.

The next day, they go to see Mary’s odd Uncle Albert, who has floated up in the air due to his uncontrollable laughter. They join him for a tea party and try to get him to stop laughing so much.

Afterward, Mr. Banks becomes annoyed by the household's cheery atmosphere and threatens to fire Mary, but she persuades him to take the children to his workplace the next day. That evening, Mary sings to the children a hymn-like lullaby about the old woman who sits on the steps of St Paul's Cathedral selling bird food.

The next day at the bank, the children meet the elderly Mr. Dawes Sr, who urges Michael to invest his tuppence in the bank, before snatching the coin from Michael. Michael demands his coin back; other customers overhear the conflict, and they all begin demanding their own money back and chaos ensues. Jane and Michael flee the bank before meeting up with Bert again, now working as a chimney sweep. They find Mary and venture onto the rooftops, where they get to sing and dance with all the other chimney sweeps. Mr. Banks

then gets a phone call from the bank requesting a meeting with him about what the children did. The children overhear the phone call and realise that their father is in trouble. Bert tells Mr. Banks that he needs to spend more time with his children before they grow up. Michael gives George the tuppence, hoping to make amends.

Mr. Banks goes to the bank, where he is dismissed. Looking to the tuppence for words, he blurts out "Supercalifragilisticexpialidocious” which seems to make him feel so much better!

The next day, the wind changes, meaning Mary must leave. After finding out from Mr Dawes Jr that he has been re-employed by the bank as a Junior Partner, an ecstatic Mr. Banks is at the park, having fixed his children's kite and taking the family out to fly it.

Mary watches the family. With her work done, she opens her umbrella and it carries her away on the wind. Bert smiles and tells Mary Poppins not to stay away too long.