Amy Castle was born on 9 May 1880 at Maori Gully, near Greymouth, the seventh of eleven children of Ellen Wilson and her husband, Henry Samuel Castle, a storekeeper and former gold prospector. Her mother died in 1894 and by 1896 her father had moved to Reefton, where he worked as an accountant for Consolidated Gold Fields of New Zealand. He was later secretary of the company.
Little is known of Amy's early life. By 1907 she lived in Wellington and that year was employed by the Dominion Museum as a temporary assistant in the photography section. In July she was appointed to the permanent staff as a photographic assistant. The practice of employing female assistants, who were paid less than their male colleagues, was seen as one way of saving money at a time when the museum was under-resourced and badly housed in its first location behind Parliament Buildings in Wellington. The building was damp and poorly ventilated, with no provision for heating, and despite the director, Augustus Hamilton, continually asking the government for more money, working conditions for staff continued to be extremely difficult.
Amy Castle was subsequently transferred to work on the entomological collections under Hamilton's supervision. She was the first entomologist and the first woman appointed in a professional role in a New Zealand museum. She was also one of the first women to be employed in a scientific position in the New Zealand public service. Following Hamilton's sudden death in October 1913, she assumed complete responsibility for the collections. There were 6,000 specimens of Lepidoptera alone, and until 1915 one of her main tasks was to catalogue, remount and reorganise it. Even so, she earned two-thirds of the salary of a male co-worker of equivalent status.
Amy Castle sought to increase enthusiasm in the wider community by encouraging budding entomologists. In what can be seen as the beginnings of the museum's education service, she taught hundreds of schoolchildren who visited the museum with their teachers. In 1915 she arranged for insect killing-bottles and capturing-nets to be issued to children, with instructions on how to do their own insect collecting, but was disappointed that this initiative did not result in a flood of entomological donations to the museum. In 1918 she reported that 'a small class of boys has been formed, who show a keen desire to study the life-history of butterflies and moths'. They attended weekly meetings to record and arrange their captures. To foster the development of interest in entomology among adults, Castle gave public lectures. Following one at the Wellington Lyceum Club in 1927, the Evening Post reported that 'Wellington is fortunate in having so gifted a lady in charge of this branch of research work'.
Overall, her work at the museum was extremely varied and the tasks of preparing and classifying insects for the collection were substantial. While she was employed at the museum the size of the entomological collection increased considerably. In 1927, for example, the Fenwick Collection of over 8,000 insects was acquired. Castle also went on a number of collecting expeditions around New Zealand. In 1927–28 she visited Mt Egmont, Kapiti Island and the Remutaka Range to collect specimens. Her primary research interest was moths and butterflies, on which she published five short articles in the New Zealand Journal of Science and Technology in the early 1920s. One of these was written jointly with Johannes Andersen and Alfred Philpott. She was a fellow of the Entomological Society of London.
Amy Castle's career at the Dominion Museum continued until April 1931, when staffing levels were cut as part of a wider government effort to reduce public spending during the depression. She never married, and lived in Wellington until 1957, when she left for England. She died at Paignton, Devon, on 23 February 1971.