Miss Andrew makes her appearance at the beginning of act two of the musical. Jane and Michael are the children of the wealthy bank officer George Banks in London. The children are naughty and have driven off every nanny their mother has hired with their antics. The strange nanny Mary Poppins appears to take care of them and teaches them many lessons about life through fun, song, and a little magic. However, the children turn out to be tough charges even for her, so she leaves for a while to teach them a lesson.
In her absence, a distraught Mrs. Banks hires Miss Andrew, her husband's old nanny. George Banks had spoken of her with praise several times earlier, and Mrs. Banks only wishes to please her husband. Miss Andrews, an old woman in black, arrives at the stage accompanied by lightning and thunder. She immediately establishes dominion over the household with intimidation, singing of her tyrannical way of rearing children, and forcing large doses of her tonic in the children's mouths. She demands Michael go to boarding school immediately and wants to take charge of Jane personally.
However, the terrified children run away from home again. This distresses Mrs. Banks, who suspects she has made a grave mistake. This also causes her to see why her husband turned out as he did, emotionally distant and obsessed with precision and order. Miss Andrew abuses the rest of the servants while awaiting the children's return. She trusts in cold and hunger driving the children home in time, and intends to feed them large doses of her tonic when they do.
Meanwhile, the children meet a good friend, Bert, in the park, who lifts their spirits. Having learned their lesson from Miss Andrew's reign of terror, the children are reunited with Mary Poppins, who agrees to return to their house and stop Miss Andrew. Upon coming home, the trio discovers how she treats the servants. Mary Poppins, however, is more concerned about Miss Andrew's trapped lark, Caruso. It has been sitting in the cage for years, and it is implied Miss Andrew treats it very badly.
Mary Poppins frees the lark, seconds before Miss Andrew arrives. Upon seeing the children, she gleefully fetches her "Brimstone and Treacle" bottle, but then notices Mary Poppins, who tells her to pack up and leave immediately. It turns into a musical standoff between the two nannies as they preach their way of treating children. Enraged with Caruso being freed, Miss Andrew threatens to punish them all, but Mary Poppins exposes her as the tyrant she is and gives her a dose of her own medicine.
Miss Andrew is forced to down the entire bottle of "Brimstone and Treacle" and is trapped in a birdcage, which vanishes into the ground in a lot of smoke and red lights, presumably to Hell. However, in the tour version of the show, a large birdcage comes out of the broom closet. Miss Andrew gets trapped and dragged into the closet with a cloud of smoke. The door slams shut and when Jane opens it up again to look, Miss Andrew and the cage are gone. In other versions of the play, Mary Poppins sends Miss Andrew spinning out the door, blown away in the sky, or is kicked out of the window.