Pérrine Millais was born in London, England, on 8 February 1893, the daughter of Mary St Lawrence Hope-Vere and her husband, Everett Millais, a medical student at the time who died in 1897. Her paternal grandparents were the British painter Sir John Millais and his wife, Euphemia Gray, who had been married to John Ruskin. Pérrine was educated by European governesses and at two London secondary schools before studying music and languages in Brussels. She became interested in conservation and ornithology during her childhood and was encouraged by her uncle, the naturalist John Guille Millais.
While holidaying in Switzerland, Pérrine met Malcolm Matthew Moncrieff, a retired British Army officer. The couple were married in London on 3 June 1914 and would have two sons. After the First World War the family lived in Scotland. In 1921 they left Britain to settle in Canada, but first visited New Zealand and decided to stay.
They settled in Nelson, attracted by its beauty and climate, and by the wealth of its bird life and flora. Pérrine was soon travelling and tramping throughout the region observing the native birds. In 1923 she became a foundation member of the New Zealand Native Bird Protection Society and around the same time joined the Royal Australasian Ornithologists' Union (RAOU).
There was no contemporary pocket field guide on native birds, and in 1925 Pérrine wrote New Zealand birds and how to identify them. She used her own observations in the field and in museums, ornithological texts, and had help from R. A. Falla, W. R. B. Oliver and other ornithologists. Although she intended her book for the untrained bird-lover, it influenced scientists as well as lay people and ran to five editions.
Pérrine Moncrieff went on to contribute papers to RAOU's journal, the Emu, and articles to Birds and the Nelson Evening Mail and other newspapers. She also gave public lectures and papers to the Nelson Philosophical Society. For at least 25 years in the Nelson Evening Mail, she summarised the best entries in the annual children's nature diary competition, which she initiated in 1928. One of her most influential papers, 'The destruction of an avian paradise' in the journal of the Society for the Preservation of the Fauna of the Empire in 1944 publicised overseas the diminishing habitat of New Zealand native birds. The paper embodied the values she encouraged: an ecological attitude to and love of nature, a wish to preserve the past for the future, and an appreciation of Romantic literature about the natural world.
After the death of their elder son in 1925, the Moncrieffs donated part of Haulashore Island to the people of Nelson. They later purchased a large tract of coastal bush at Okiwi Bay and presented it to the Crown. Pérrine was involved in many campaigns to save native bush and bird species, especially as a member of the Nelson Bush and Bird Society, which she established in 1928. Among its achievements, the group helped make Lake Rotoroa a scenic reserve and Farewell Spit a sanctuary. Pérrine's most notable success was the establishment of the Abel Tasman National Park in 1942. She served on the park board from 1943 to 1974 and published a book on the history of the area, People came later, in 1965. As a lobbyist she was meticulous in background research into legislation and land ownership and was careful to secure and maintain the support of influential politicians and public servants.
Pérrine Moncrieff became a vice president of the New Zealand Native Bird Protection Society in 1927 and was president of the RAOU in 1932–33: the first woman to hold this office. Her presidential address, 'Birds in relation to women', included a condemnation of the practice of killing native birds to adorn women's dress, advocating instead the use of feathers from birds killed for human consumption. She wore a cap of white hens' feathers dyed sapphire blue to illustrate her point.
Pérrine deplored the further killing of birds for private or museum collections when species were already well represented. She also believed that sufficient habitat should be reserved to allow birds to breed successfully in the wild, rather than in captivity. Convinced that the aims of the Ornithological Society of New Zealand were inconsistent with these beliefs, she resigned her membership in 1942, two years after helping to establish the society. Pérrine kept a pet macaw, called Miss Micawber, for 30 years. She was known locally as the 'bird woman' and all sick birds and enquiries were notified to her.
She was president of the Nelson Institute and of the Nelson Philosophical Society. As well as starting the Girl Guides movement in Nelson, she was a justice of the peace and an honorary ranger for the departments of Internal Affairs and Lands and Survey. She formed the Nelson Spinners' and Weavers' Guild during the Second World War and was a working member of the local arts society. A member of many nature-centred organisations, Pérrine remained an enthusiastic and persistent advocate for conservation. In her 80s she was involved in the campaigns of the Native Forests Action Council and the Friends of Nelson Haven and Tasman Bay.
Pérrine was awarded the Loder Cup in 1953, appointed an officer of the Dutch Order of Orange-Nassau in 1974 and was made a CBE in 1975. In the following year, she published The rise and fall of David Riccio, a historical novel set in Scotland. A small woman with authority, charm and humour, Pérrine Moncrieff died at Wakapuaka, near Nelson, on 16 December 1979. Malcolm Moncrieff had died in 1968.