Robert McNab was born at Dunragget farm, near Invercargill, New Zealand, on 1 October 1864. He was the eldest child of Janet McQueen and her husband, Alexander McNab, a runholder who served as a member of the Otago and Southland provincial councils. By early 1871 Robert was left as the only child in his family after five brothers and a sister had died. He was educated at the Invercargill District High School, where he was dux in 1879. He enrolled at the University of Otago in 1880 and graduated BA in 1883 (when he was also awarded a Senior Scholarship), MA with second-class honours in mathematics and mathematical physics in 1885, and LLB in 1891. In 1886 he entered the office of Smith, Chapman, Sinclair and White as a law clerk and was admitted to the Bar in 1889. Both in Dunedin and, later, in Invercargill he served in the volunteers and was a keen and accomplished rifle shooter.
In 1890 McNab went into legal practice in Invercargill with J. L. M. Watson. This lasted until 1896, when he went farming on the family's Knapdale homestead block in the upper Mataura River valley. McNab was an advocate of scientific methods of farming and pioneered managed farm forestry at Knapdale in the 1890s. He later founded a Romney stud, introduced a shorthorn herd and imported the area's first farm tractor. His reading and practical experience were embodied in four newspaper articles reprinted in 1903 as a pamphlet, Forestry in its relation to the farmer.
In 1891 McNab was elected to the Southland Education Board and in 1892 to the Southland Boys' and Girls' High Schools Board. He promoted Saturday training classes for teachers in Invercargill and became president of the Southland branch of the New Zealand Educational Institute.
McNab entered national politics in 1893 when, as a Liberal, he won the Mataura seat from G. F. Richardson, a former cabinet minister. He lost it in 1896, but was re-elected in 1898 on Richardson's retirement from politics and retained the seat until 1908. He was twice offered a post in the Seddon ministry but his conditions – the restructuring of the cabinet – were unacceptable to Seddon. After Seddon's death he became minister of lands, minister for agriculture, minister in charge of the Valuation Department and commissioner of state forests. His 1906 bill to strengthen leasehold land legislation and to set aside Crown land for a national endowment fund met strong opposition from advocates of freeholding, including a substantial number of his own party; it was withdrawn by Prime Minister Sir Joseph Ward. The leading role McNab took as a government spokesman on the issue, and his responsibility for the 1908 Dairy Regulations, disparagingly known as the 'cow tax', contributed to his defeat in 1908.
Out of Parliament, McNab in 1909 began a very successful national speaking campaign, at his own expense and without the support of the Liberals, for the introduction of compulsory military training in the Defence Act 1909. While on a visit to Britain in 1910, he was offered the Liberal candidacy for the seat of North East Lanark; he declined the offer, but thought seriously about entering politics in Britain if he failed to be re-elected in New Zealand.
In 1911 he stood unsuccessfully for the Palmerston North seat and in 1914 won Hawke's Bay, which he held until his death. In 1915 he reluctantly entered William Massey's wartime National ministry as minister of justice, minister of marine, minister of stamp duties, and minister in charge of tourist and health resorts.
McNab was an early advocate of electoral reform, writing a series of articles in the New Zealand Times in 1913, which were reprinted as a pamphlet, Open letters on proportional representation. He introduced to Parliament several private member's bills on the subject. From 1895 to 1904, and from 1908 until his death, he was one of the vice presidents of the prohibitionist New Zealand Alliance, and he was a lifelong member of the Presbyterian Church. From 1911 to 1914 he was a member of the Victoria College Council.
The 1890s had seen an increasing self-consciousness about New Zealand's distinct identity. With J. A. Hanan, McNab founded the Invercargill branch of the New Zealand Natives' Association in February 1898. About the same time he began his researches into the early history of New Zealand. The first fruits were 12 articles in the Gore Southern Standard on events in the history of Southland. After visits to Australian libraries these and 13 more articles were reprinted as Murihiku: some old time events in 1905. In 1906 McNab completely recast the material, but after further discoveries in American whaling ports he published a substantially revised version, Murihiku and the southern islands…from 1770 to 1829, in 1907.
Between 1904 and 1907, during parliamentary recesses, McNab travelled overseas to Hobart, Sydney, the United States and Britain in search of manuscripts, archives and newspaper accounts which he transcribed; he corresponded with institutions in Paris, Madrid and St Petersburg. In 1909, after further overseas visits, a new edition, Murihiku: a history of the South Island of New Zealand, was published.
McNab's assiduous gathering of primary sources for New Zealand's early history did not go unnoticed and he was invited by the government to edit the material for publication. The Historical records of New Zealand, modelled on the 1892 Historical records of New South Wales, appeared in two volumes. The first, in 1908, comprised almost exclusively the New South Wales official manuscript and printed records relating to New Zealand; and the second, in 1914, substantially the logs and journals of Tasman, Cook and other exploring expeditions in New Zealand waters up to 1839.
In 1911, when he was resident in Palmerston North, McNab published the results of his new researches on the North Island in a long series of articles in the Manawatu Daily Times; they dealt with the early history of Cook Strait. The old whaling days was published in 1913, and included additional material gathered after the publication of the 1909 edition of Murihiku. In 1913 McNab submitted Murihiku and The old whaling days as a thesis and was awarded the degree of LittD by the University of New Zealand in 1914. He had been elected a fellow of the Royal Geographical Society, London, in 1908. From Tasman to Marsden, which appeared in 1914, was a further addition to McNab's announced project to cover New Zealand's history between 1642 and 1840.
Robert McNab's historical method was unashamedly that of the compilation historian, unearthing fragments of information and documents from a painstaking search of the primary sources and presenting them in a chronological narrative. He was not interested in constructing 'literary edifices': 'The reader is given the results of the Author's research, not the fruits of his thought.' He justified his method of publishing by the need to make the information available to the public as soon as possible; to wait until everything had been collected would be to postpone publication indefinitely.
In December 1913 McNab donated his historical library of some 4,200 volumes to the Dunedin Public Library to provide the city with a supplement to T. M. Hocken's collection in the Otago University Museum. It was gifted on condition that the collection was added to, and he subsequently lodged copies of transcriptions and notes for some of his earlier published works. The bulk of his surviving papers is in the Alexander Turnbull Library, Wellington.
Robert McNab was a tall, thick-set man, shy and reserved, whose commitment to public service was evident early in his career. He lectured throughout New Zealand on aspects of his researches and was a capable speaker and debater, but lacked the skills of the popular orator. His political career was marked by a concern for the right rather than the expedient or the popular, and by diligent application. His contemporaries concurred on his caution, fair-mindedness, competence as an administrator, appetite for hard work and mastery of detail – and his lack of personal magnetism and leadership.
McNab's major contribution to New Zealand lay in documenting publicly the early years of European contact and in giving New Zealanders a sense of the extent of this contact prior to 1840. He was, in his writings, a patriot, one of the small band of men and women, mostly New Zealand born, who came to maturity and positions of influence around the turn of the century and helped to create in history, science, literature, the arts, journalism and politics a sense of a distinctive New Zealand destiny and identity for those of European descent.
McNab never married. He died in Wellington on 3 February 1917 of heart failure after a severe attack of pleurisy and was buried in Invercargill.