Martha Rendell moved in with Thomas Nicholls Morris at 23 Robertson Street, East Perth after he had separated from his wife. His wife had left and was living somewhere else. Morris had custody of his four children. Rendell, who had known Morris from Adelaide, South Australia, followed him to Perth and moved into his house. She pretended to be his wife. The children were told to call her "Mother."

Rendell was very cruel to Morris' children. She once beat Annie so badly that Annie couldn't walk. The officer who arrested her, Inspector Harry Mann, said that Rendell enjoyed watching her victims suffer. He believed she got some kind of pleasure from seeing them in pain.

Martha Rendell's first victim was nine-year-old Annie. Rendell would put something in the children's food that made their throats sore. She would then swab their throats with hydrochloric acid, pretending it was medicine. This caused their throats to swell until they couldn't eat, leading to starvation. Annie died on July 28, 1907. The doctor, Dr. Cuthbert, believed the cause of death was diphtheria and issued a certificate saying so. After Annie, Rendell targeted seven-year-old Olive. Olive died on October 16, 1907, and again, Dr. Cuthbert said it was diphtheria.

In the winter of 1908, Rendell used the same method on Arthur, the youngest child. Arthur was 15 and took longer to die from the treatment. He finally passed away on October 8, 1908, nearly a year after Olive's death. This time, Dr. Cuthbert asked for an autopsy. Rendell wanted to be there during the investigation, and nothing suspicious was found during the autopsy.

In April 1909, Rendell started to target the second son, George. After drinking a cup of tea, George complained of a sore throat. Rendell coated his tonsils with the acid, scaring the boy. George ran away to his mother's house. When neighbors asked about George, his father, Thomas Morris, said he didn't know where George was.

Neighbors went to the police, and Inspector Harry Mann started investigating. Mann heard about the children having their throats painted and Rendell not caring about their pain. One neighbor said he saw Rendell rocking back and forth in front of a screaming child, looking pleased. Some even claimed to have seen her doing inappropriate things. Mann found George, who said he ran away because Rendell had killed his siblings and was trying to poison him too. The investigation was hard because of the time that had passed and the lack of clear evidence about the effects of the acid. Suspicion grew when it was found that Rendell had bought a lot of hydrochloric acid during the children's illnesses but stopped after their deaths. The police got permission to exhume the bodies, and on July 3, 1909, they did. They found diluted hydrochloric acid on the children's throat tissues.

The trial for Rendell and Morris started on September 7, 1909, in the Supreme Court of Western Australia. Neither of them defended themselves. On September 14, 1909, the jury found Rendell guilty of murder, and she was sentenced to death by hanging. Rendell claimed she was innocent and said she was treating the children for diphtheria. Thomas Morris was found not guilty. It was believed he didn't know what Rendell was doing until after the children died. The jury wanted to find him guilty of being an accessory after the fact, but they couldn't.

Rendell's crimes caused a lot of public anger. The press called her a "scarlet woman" and a "wicked stepmother." Despite claiming her innocence, Rendell was hanged at Fremantle Prison on October 6, 1909, at 8 am. Her execution was witnessed by 25 people, the most for any execution at Fremantle, even though the Sheriff did not allow the press to attend. She was buried at Fremantle Cemetery, in the same grave where serial killer Eric Edgar Cooke was buried years later. Martha Rendell was the third and last woman to be executed in Western Australia.

An illusion of Rendell's face appears on one of the prison windows. It can only be seen from the outside; inside, the glass looks normal. This is an example of pareidolia, and the legend says Rendell's spirit watches over the prison.

Martha Rendell's crimes were featured in an episode of the true crime TV series Deadly Women called "Pleasure From Pain," Season 5, Episode 14.