While Vivie is still gone from the office, Mrs. Warren arrives unannounced. She is worried that Vivie won't want to see her, but Praed pretends not to know what she is talking about. Frank finishes writing his farewell note to Vivie before turning his attention to her mother.
Frank tells Mrs. Warren that she ought not to wait for her daughter's return, but won't be specific about why. Mrs. Warren starts crying because she doesn't understand why Frank would advise her to leave. Praed agrees with Frank, but it is too late: they hear Vivie returning.
Vivie tells Praed and Frank to leave and asks her mother to sit down. Mrs. Warren asks why she has returned her monthly allowance check. If it is not enough money, she says, she will give Vivie whatever amount she asks for. Vivie refuses to follow that false line of reasoning, telling her mother that she will no longer accept any money from her and in the future, she will support herself.
Mrs. Warren pretends to not be sure why Vivie is cutting ties with her. Vivie quickly tells her that Crofts told her everything. She makes the point that she had been sympathetic to the history her mother shared with her, but the fact that Mrs. Warren is still involved in prostitution changes their relationship.
Mrs. Warren tries to convince Vivie not to end their relationship, explaining how wealthy she is and the type of life Vivie can live with her. Vivie recognizes this as the same type of argument Crofts made to her in Act III, and assures her mother that she wants nothing to do with the lazy lifestyle of the very wealthy.
Mrs. Warren becomes emotional and lapses into her native accent, begging Vivie to take care of her when she grows old. She claims that she has the right to Vivie's companionship as a mother. Vivie, on the other hand, has her own interests in mind and denies her mother the relationship she begs for.
Before leaving, Mrs. Warren says she regrets the way she raised Vivie to be independent and educated, and wishes she had not tried to be a good woman her whole life. Finally, she agrees that Vivie is right to end their relationship and leaves, slamming the door. Vivie reads Frank's note, chuckles before throwing it in the trash, and returns to work.
The relationship between mother and daughter is fraught in this act. Vivie cites the similarity between them as the very reason they must part ways: "I am my mother's daughter. I am like you: I must have work, and must make more money than I spend" (102). Despite that, they differ on a deep moral level in terms of what they believe is right and wrong.
For Vivie, morality is dependent on being true to oneself. She denies marriage proposals from Frank and Crofts simply because she does not want to be married. She denies a relationship with her mother because it would not make her happy. She works all the time rather than taking opulent vacations because she prefers it. This is unconventional, and she embraces it. In contrast, she accuses her mother of being "a conventional woman at heart" (104), despite her unconventional profession.
The contrast between Mrs. Warren's sentimentality and Vivie's practicality is obvious from the moment Mrs. Warren enters the office. She is crying but, in a moment of self-awareness, attempts to hide it from Vivie. She tells Frank and Praed, "She'll be so angry if she sees I've been crying," (98) and begs them not to tell her daughter. She has learned, since their first interaction at Haslemere, that tears do nothing to move Vivie's emotions. If anything, displays of emotion annoy her daughter.
The idea of morality upheld by the wealthy arises again in the final conversation between Vivie and Mrs. Warren, with Vivie pointing out how false it is. She decries the pretense of "fashionable morality" (102) and calls those who aspire to it "worthless" (102). This undermines Mrs. Warren's first argument as to why Vivie needs her, since Vivie has no interest in the lifestyle her mother's money can afford.
The central conflict of Mrs. Warren's character is that she cannot reconcile her ideas about right and wrong. Vivie points this out when Mrs. Warren declares that she wishes she had not tried to "be a good woman" (104) her whole life but rather embraced what she sees as the inherent wrongness in her profession. Her mother "should not have lived one life and believed in another" (104), but rather committed to one line of reasoning.