Grills was born Caroline Mickelson in 1890 in Balmain, Sydney. She was a plump, cheerful, much-liked woman of sixty-three at the time of her arrest, in the habit of visiting relatives and making them tea.
Between 1947 and 1953 four people close to her died: her stepmother Christine Mickelson, eighty-seven; Angelina Thomas, eighty-four; John Lundberg; and Mary Ann Mickelson. She used thallium, a heavy metal then sold freely in Australia as Thall-rat, a rat poison, and available in any shop. Thallium poisoning produces hair loss, nerve damage, blindness and a slow death that in the elderly was readily taken for decline.
Suspicion arose in 1953 when relatives noticed that a niece by marriage, Christine Downey, and her husband deteriorated after Grills's visits. A family member watched her put something into a cup of tea, and the tea was analysed. Thallium was found.
She was tried in October 1953 for the attempted murder of Christine Downey, a single count, and the only one prosecuted, convicted, and sentenced to death on October 15, 1953. The sentence was commuted to life imprisonment. She died in Long Bay gaol on October 6, 1960. Other prisoners called her Aunt Thally.
No motive was ever established. She gained nothing material from most of the deaths, and the prosecution offered no account of why she did it. The case belongs to the Sydney thallium cluster of 1952 and 1953, a run of poisonings that led directly to the removal of thallium from open sale in New South Wales. One conviction, for an attempt, is what the record establishes; the four deaths are attributed.