Like Jane, Michael had possessed a naughty streak which caused the resignation of 6 nannies in four months. However, upon Mary Poppins' arrival, it appears that he isn't as bad as is said. The tape measure describes him as extremely stubborn and suspicious. He also seems to be friendly and playful.
At the beginning of the film, he is distant from both his parents. He seems to feel insecure due to failing to live up to his father's ridiculously high standards for him, his mother and sister, and mistakenly believes that his father hates him, though this changes by the end.
Michael's first appearance is when he and Jane are brought home by Constable Jones. They had been separated from their nanny, Katie Nanna, after having trouble with their kite. She insists it was intentional and resigns as a result, but from their perspective, it was merely an accident.
Later, when George and Winifred are drafting an advertisement for a new nanny, Jane and Michael bring their own. Theirs suggests that they have played several pranks on their former ones, but also calls for a fun one who will love and play with them. The more practical George dismisses the ad as ridiculous, tears it up, and tosses the pieces into the fire.
The next morning, Michael is seen watching the nanny applicants, all stern old women, who have responded to George's ad for a tough, commanding one. Much to his surprise, a sudden wind blows them all away. Jane and Michael notice a woman that fits their ad, flying by using an umbrella. Michael initially believes her to be a witch, but he is corrected by Jane, who says that witches have brooms.
Later, the woman comes to the nursery and introduces herself as their new nanny. After being amazed by her bottomless carpet bag, (though Michael crawls under the table to examine it), she measures both of them with her tape measure. It says that Michael is "extremely stubborn and suspicious." After he requests that she be measured, he learns that her name is Mary Poppins. She then leads them in a game, tidy up the nursery. Both Jane and Mary are able to snap their fingers, causing the toys to put away by themselves. Michael, however, can't snap his, and when he finally does, he ends up in the closet, unable to get out because the doors keep opening and closing themselves. He enjoys the "game" anyway.
Then Mary takes the children on an outing in the park. They intend to go there, but they run into Bert, an old friend of hers. He tells the children about her magic and is able to convince her to transport them into his picture of the English countryside. While he and Mary enjoy a walk through the countryside, Jane and Michael enjoy a nearby fair. They meet up at Mary's private carousel, where she causes the horses to jump off. In a bit of competitiveness, Michael makes his go faster than the rest, before being restrained by Mary. Eventually, they end up at a race course, where Michael witnesses Mary effortlessly win the race. He learns of her magic word, Supercalifragilisticexpialidocious, but they are soon forced to leave when a rainstorm hits, which washes the drawing off. That night, Mary puts the children to bed and denies that anything unusual happened.
The next day, Mary, Jane, and Michael are out to do errands, but are called to the assistance of Mary's Uncle Albert by a dog named Andrew (she is able to understand him, though Michael believes he said nothing). They go to the house to find Bert already there. The children discover that Uncle Albert "suffers" from a condition where he uncontrollably floats to the ceiling when he laughs. Despite being warned by Mary, the condition spreads to Bert and the children. At Uncle Albert's request, they stay for tea after Mary makes the table float into the air. Eventually, everyone comes down at the sadness of being told they must leave.
That night, George attempts to fire Mary after hearing the children's unbelievable tales. However, they are happy to find that she has not been fired. Michael is puzzled, however, when she announces they will go on an outing to the bank with their father, as he had never taken them anywhere. Mary tells them about the Bird Woman, who allows people to feed the birds for tuppence nearby.
Michael goes with his father and sister to the bank and secretly brings tuppence to feed the birds. George refuses to let him, however, and attempts to convince him to invest it in the bank. He is almost convinced but is angered when the owner, Mr. Dawes Sr., takes it without asking. His attempt to retrieve it begins a run on the bank.
In the chaos, Jane and Michael escape from the bank. They become lost but soon run into Bert, who is working as a chimney sweep. They reveal the trouble at the bank but do not seem to understand what went wrong, though they believe that their father hates them for it. He helps them to understand that George does not hate them, but is merely trapped in a cage by his responsibilities. He takes them home and is unwittingly enlisted to watch them by Winifred, as it is Mary's day off. He allows them to help him clean the chimney. As they are working, Mary returns and warns them of the danger, just as Michael is sucked up it. Jane, Mary, and Bert follow, and Mary leads the group on an expedition of the rooftops. They run into Bert's chimney sweep friends, and party all the way back to the Banks' living room. Bert runs out the sweeps when George returns home, who catches Michael trying to leave with them.
After hearing that his father is in trouble for the events at the bank, Michael gives the tuppence to him, in the belief that it will help fix things. The next morning, Mary is preparing to leave, much to Jane and Michael's disappointment. However, they soon hear their father calling them, though he doesn't believe it to be him, as the voice is happy. They head downstairs and discover that he has fixed their broken kite. During the night, he had a change of character and had decided to become a more involved father. He takes his family kite flying. Michael is so happy to be with his father, that he forgets to say goodbye to Mary, though she notes that that is as it should be. He is last seen happily flying his kite alongside his family.