Alexander Horsburgh Turnbull was born in Wellington, New Zealand, on 14 September 1868, the sixth child and youngest son of Walter Turnbull, merchant, and his wife, Alexandrina Horsburgh. Turnbull's parents, born in Scotland, had arrived in Wellington in 1857. In February 1875, after a successful business career in New Zealand, Walter Turnbull took his family to settle in London. Alexander had had two years of schooling in Wellington (probably at Thorndon School in Sydney Street), and then attended Dulwich College, London, from April 1881 to 1884. His academic record was undistinguished apart from a prize for mathematics in his first year. He was a member of the shooting team for two years and of the school's first rugby fifteen in his final year.
In 1884 Alexander began work in the London office of the family firm of Turnbull, Smith and Company, wholesale drapers. In 1885 and again in 1886 he paid extended visits to New Zealand, and just before the second visit purchased, in late 1885, a copy of J. H. Kerry-Nicholls's The King Country; or, explorations in New Zealand (London, 1884); it was the first book of his collection. He had had a coin collection since the age of eight.
In May 1888 Turnbull, Smith and Company was sold to Sargood, Son and Ewen of Dunedin, and Alexander Turnbull was free to indulge himself as a young and wealthy man-about-town in a wide-ranging social life, travel, reading, developing his bibliographical knowledge and skills, collecting, and establishing his relationships with London and provincial booksellers. His lifelong concern with the contents of his books is shown in his additions (begun in 1889) to J. D. Davis's bibliography of New Zealand, and his compilation of notes on New Zealand place names, begun in 1890. By 1891 most of the major, and some minor, themes of Turnbull's collection were well established: New Zealand, Pacific exploration, Scottish history, English literature, John Ruskin, and the fine arts. The collection was valued for insurance by the London bookseller Bernard Quaritch at £1,500 in that year.
In early 1892 Alexander Turnbull returned to Wellington to join W. & G. Turnbull and Company, the firm of general merchants founded by his father. Turnbull, his parents and his younger sister Joanna re-established themselves in Elibank, the family home on Bowen Street. In August 1892 he became a partner in the firm, joining Nicholas Reid and J. H. Cock.
Turnbull's collecting now became more purposeful. In July 1892 he wrote to Quaritch of his intention to form a Milton collection. His collecting of Māori and Pacific artefacts and clothing increased substantially. In May 1893 Turnbull committed himself to building a comprehensive collection relating to New Zealand: 'Anything whatever relating to this Colony, on its history, flora, fauna, geology & inhabitants, will be fish for my net, from as early a date as possible until now'. At the time he possessed some 1,500 volumes relating to New Zealand. Between 1894 and 1895 he returned briefly to collecting coins.
The pattern of comprehensive collecting in well-defined subject areas was strengthened during the 1890s with commitments to ocean voyages and naval history in 1896; James Cook in 1896; and the 'Australasian Colonies & South Sea Islands' in 1899. The only new subject area was English drama, which Turnbull began developing in 1915. In 1913 he made an anonymous donation of his substantial collection of Māori and Pacific Islands artefacts, some 500 items, to the Dominion Museum.
W. & G. Turnbull and Company became a limited liability company in 1913, and was sold to Wright Stephenson and Company of Dunedin in 1916. Turnbull retired from business but remained heavily involved in the family's difficult financial affairs. He died in Bowen Street Hospital in Wellington on 28 June 1918 after an operation on his sinuses. Turnbull never married; he was survived by his sister, Joanna (later Lady Leigh-Wood) and his brother, Robert Thorburn Turnbull, founder of the firm Turnbull and Jones.
Turnbull's recreations were golf and yachting. He was a member of the first committee of the Hutt Golf Club in 1892, and commodore of the Arawa Sailing Club and the Port Nicholson Yacht Club. He owned and sailed the Rona, the Miru and the Iorangi between 1893 and 1912. Turnbull's sole publication was the Account of a cruise in the yacht 'Iorangi', to Queen Charlotte Sound, New Zealand (1902). He was elected a fellow of the Linnean Society of London in 1891, was a fellow of the Royal Geographical Society, joined the Polynesian Society in 1893, and belonged to the Lyric Club (London), the Wellington Club, and the Wellington Chamber of Commerce. He served as honorary vice consul for Spain from 1899 to 1913.
Alexander Turnbull's library, housed since 1916 in his specially designed home in Bowen Street, was valued at £51,800 in 1918. It consisted of some 55,000 volumes of books, pamphlets, periodicals and newspapers, and thousands of maps, paintings, drawings, prints and manuscripts. It was the largest private library in New Zealand, and was of international standing particularly for its Milton, New Zealand and Pacific collections. Turnbull bequeathed his collections to the government to constitute a reference library in the city of Wellington, the contents not to be lent out but kept together 'as the nucleus of a New Zealand National Collection'. Turnbull's home was purchased by the government to house the collection, and on 28 June 1920 the Alexander Turnbull Library was opened to the public as a national reference and research library.
Alexander Turnbull was a man of two worlds: a dandy and aesthete, at home in the cultural and social world of fin de siècle London; a New Zealander by birth and, from 1892, by commitment, although never at home in colonial society; and this double allegiance characterises his collecting. As a book collector in the European tradition he selected from well-established subject areas to document the high culture of the Old World: the complete works of a major English author (Milton), English literature, the voyages and travels of the European peoples, private presses and fine printing; he emphasised first editions, fine condition, and fine bindings, to provide examples of the printed book as a characteristic artefact of European culture. As a colonial collector, sensitive to the nationalism of the 1890s, he committed himself to the creation of a national collection of everything relating to New Zealand and its environs, to document the creation of a new society in the south-west Pacific, and to serve the first generation of indigenous scholars, his colleagues and friends Robert McNab, S. Percy Smith, John Macmillan Brown, W. Downie Stewart, Herbert W. Williams, and others.
Turnbull was a typical member of his generation, a pragmatist not greatly given to theorising, with a tireless persistence in pursuing his aims. His one direct statement of his aspirations, made in 1912, was that 'My books & MSS. I hope will assist future Searchers after the truth.' His memorial is the great library he bequeathed to the people of New Zealand.