In the novel Death on the Nile, Mrs Allerton was one of the passengers of the Egyptian steamer S. S. Karnak when Linnet Doyle was killed. Mrs Allerton was the mother of Tim, with whom she had been in Maiorca before traveling to Egypt. She recognised Hercule Poirot at the Cataract Hotel in Assuan, and invited him to dine with her on the Karnak.
Mrs Allerton is described as a "good-looking, white-haired woman of fifty". She feels intense affection for her son, and seeks to disguise this by "imparting a look of pinched severity to her mouth" when she looks at him. However, total strangers are seldom deceived by this, and Tim sees through it perfectly.
Mrs Allerton is irritated by the friendship between Tim and his second cousin, Joanna Southwood. This is not because she is afraid that Tim will fall in love with Joanna, but because of some other feeling which is difficult to define, perhaps unacknowledged jealousy.
She thinks highly of Sir George Wode. Tim suggests that this is probably because he had called her a “rosebud” when seeing her at a dance in eighteen seventy-nine. Mrs Allerton's reply was that she hadn’t even been born in 1879.
At the beginning of the cruise on the Karnak, Mrs Allerton tries to match the names on the passenger list to the people in the dining room, saying that she always thinks this is rather fun. She also plans to talk to Miss Van Schuyler, saying that she will pave the way by talking about any title relations and friends she can remember, such as Tim's second cousin once removed, the Duke of Glasgow.
When the passengers of the Karnak go up to the rock overlooking the Second Cataract, Mrs Allerton chooses not to go on a camel "on the grounds of personal indignity". She walks up to the rock, together with Poirot.
After the death of Salome Otterbourne, Poirot asks Mrs Allerton to look after Rosalie Otterbourne. Mrs Allerton asks if there is a double cabin she can share with Rosalie, as she feels that Rosalie ought not to go back to the one she had shared with her mother. Mrs Allerton says that she is very fond of Rosalie, and that Rosalie is inclined to cling to her "in the most pathetic fashion".
Towards the end of the novel, Mrs Allerton finds out about Tim and Rosalie's romantic relationship, and is very happy. She tells Rosalie that she had hoped they would end up together, and that Tim had pretended that he did not like her, but she had seen through it.