Act 1

The Matchmaker is set in the 1880s and begins in the cluttered living room of Horace Vandergelder, a wealthy old widower living above his prosperous hay, feed, and provisions store in Yonkers, New York. His bags are packed, and he is being shaved by a barber. Ambrose Kemper, an artist, is trying to get Vandergelder to allow him to marry Vandergelder's niece, Ermengarde. Vandergelder does not approve because Ambrose does not make a steady income, and the old man is too practical to consider either love or the promise of future earnings as significant reasons to change his mind. Ambrose points out that Ermengarde is twenty-four and old enough to do what she wants. Vandergelder says that he is sending Ermengarde away to a secret place to prevent the wedding, but then his housekeeper, Gertrude, comes in and announces out loud the address where Ermengarde's luggage is being sent.

Vandergelder sends for his chief clerk, Cornelius Hackl, and explains to him that he is going away for a few days to be married. He says that he is promoting the thirty-three-year-old Cornelius to the position of chief clerk, even though, as Cornelius tells the junior clerk later, it is a position he has held for several years already.

When no other clerks are in the room, Malachi Stack enters with a letter of recommendation from a past associate. Vandergelder agrees to hire him and sends him away immediately to catch a train to New York City so that he can prepare for Vandergelder's arrival after his marriage.

Vandergelder is out of the room when Dolly Levi arrives. She is an old friend of his late wife, a matchmaker who is supposed to be finding a suitable wife for Vandergelder. She hears Ermengarde and Ambrose complaining that he is obstructing their wedding plans, and she agrees to help them, arranging to meet them at a restaurant in New York that night.

Vandergelder arrives and tells Mrs. Levi his plans to marry Irene Molloy. She makes up a story about a woman who is wealthy, socially connected, and interested in him, and so he agrees to put off proposing to Mrs. Molloy.

Left alone, Cornelius complains to the other clerk, Barnaby Tucker, that they never get time off to go out and experience life. He goes downstairs to the store and heats some cans of tomatoes until they explode, creating a foul smell that forces them to close the store, and they take off to New York, planning to have an adventure.

Act 2

In the hat shop that she owns, Irene Molloy tells her assistant, Minnie Fay, that she will marry Vandergelder if he asks, in order to get out of the hat business. She feels trapped by the reputation that milliners have, with her every move being watched by people who expect her to be a woman of low virtue. Minnie objects that Mrs. Molloy should not marry if she does not love Vandergelder.

They are in the back room when Cornelius and Barnaby come into the shop to hide, having seen Vandergelder on the street. When the women enter, the two clerks pretend to be wealthy men who are shopping for a hat—actually, "five or six"—for a friend. Cornelius falls in love with Mrs. Molloy immediately.

Seeing Vandergelder and Mrs. Levi approaching the shop, Cornelius and Barnaby hide in a closet and under the table, respectively. Mrs. Molloy suspects what is going on and leads Vandergelder to the back room to give them a chance to escape, but Cornelius decides to stay so that he can get to know Mrs. Molloy. Dolly Levi finds out about their situation and decides to help them. When Vandergelder and Mrs. Molloy come back, the conversation turns to Cornelius. Mrs. Molloy is under the impression that he is wealthy, and Vandergelder says he is just a clerk. Mrs. Levi explains that Cornelius is actually a well-known socialite, a prankster who comes from a wealthy family and works at the shop in Yonkers to amuse himself. The clerks sneeze and are found out; Vandergelder walks out, indignant, taking Mrs. Levi with him. Mrs. Molloy, thinking that Cornelius really is wealthy, insists that he and Barnaby take Minnie and her to an expensive restaurant for dinner.

Act 3

At the Harmonia Gardens Restaurant, Vandergelder plans to meet Mrs. Levi and the mysterious woman whom she said admires him. He sees Ermengarde and Ambrose enter. He pays Malachi and the cabdriver who brought them to abduct the young couple when they leave and take them to the house of Miss Van Huysen.

The two clerks arrive with Mrs. Molloy and her assistant. As she orders food and champagne, Cornelius worries about how they will pay the bill at such an expensive restaurant. The waiter sets up another table and puts up a screen between the two, for privacy; at the other table he seats Vandergelder, who is waiting for his date. Malachi finds a wallet on the floor and, not seeing that it has dropped out of Vandergelder's pocket, takes it around the screen and gives it to Cornelius, whom he has never met. No longer worried about the bill, Cornelius confesses to Mrs. Molloy that he is not rich and is just a clerk. She suggests that they just have a good time.

Mrs. Levi joins Vandergelder and explains that the woman she told him about has run away and gotten married. During their conversation, he discusses how difficult Mrs. Levi can be, and she pretends that he is flirting with her and hinting at marriage, in order to plant the idea in his head.

To get out of the restaurant without being seen by Vandergelder, Cornelius and Barnaby put on the ladies' coats and veils. Before leaving, they take time to dance. Vandergelder, dancing with Mrs. Levi, bumps into Cornelius and recognizes him. He fires both clerks, and Mrs. Molloy breaks up with him. Ermengarde enters and faints, to be carried out by Ambrose. Mrs. Levi points out the sorry situation of Vandergelder's life: "Without niece—without clerks—without bride—and without your purse. Will you marry me now?" He still refuses.

Act 4

The cab driver and Malachi arrive at Miss Van Huysen's house with Cornelius and Barnaby, who is still disguised as a woman; they have mistaken them for Ermengarde and Ambrose. Miss Van Huysen explains that she has no intention of interfering with young love as Vandergelder expects her to.

The real Ermengarde and Ambrose show up. Expecting Miss Van Huysen to object to their relationship, they tell her that Ambrose is Cornelius Hackl.

Dolly Levi arrives with Mrs. Molloy and Minnie. She pays off the cabdriver with money from Vandergelder's wallet, which Cornelius gave to her.

When Vandergelder arrives, Mrs. Van Huysen insists that he let the young lovers marry. Everyone goes to the kitchen, and Dolly Levi, addressing her dead husband Ephraim, explains that she intends to marry Vandergelder in order to spread his money around, creating happiness. When Vandergelder comes back, he does in fact propose to Mrs. Levi. Barnaby comes in and says that the other two couples are going to marry, too, and Mrs. Levi has Barnaby, as the youngest member of the cast, give a final speech to the audience about the importance of having enough adventure in one's life.