John Thomas Mair was born at Invercargill on 12 October 1876, the son of Catherine Hamilton and her husband, Hugh Mair, a carpenter, later a building contractor and mayor of Invercargill. John was educated at public schools in Invercargill and received his early architectural training with William Sharp, engineer, architect and surveyor to the borough of Invercargill. He was employed by the architectural branch of New Zealand Railways, and from 1904 to 1905 became involved with George Troup, officer in charge, in the design of Dunedin railway station during its early period of construction.
After studying at the University of Pennsylvania from 1906 to 1908 he was awarded a special certificate in architecture. He was then attached to the New York office of the architect George B. Post and in 1909 went to London, where he became an associate of the Royal Institute of British Architects. Late that year he returned to New Zealand via France and Italy, where he looked at Romanesque and Italo-Byzantine buildings. Their influence, in addition to that of the neo-Romanesque work of the American H. H. Richardson, was soon to be seen in his unusual design for the Presbyterian First Church, Invercargill. This was Mair's first major work on commencing practice in Wellington in 1910.
First Church, the cornerstone of Mair's reputation as an architect, was at first received with scepticism. Its polygonal plan and unusual positioning of choir, gallery and organ behind the pulpit were wholly unexpected. Its eclectic mixture of Romanesque and Byzantine elements was resisted by members of a Southland congregation unfamiliar with architectural fashion in cities like Boston or Philadelphia. The exterior's intricate decorative brickwork, garish to many contemporary eyes, was in fact a clever practical solution to the unavailability of other building materials.
From 1910 to 1918 Mair's work was mainly domestic. His houses were mostly in the fashionable California bungalow style, designed for middle-income clients who wanted to live in something other than the ubiquitous bay villa. For more wealthy, conservative clients he favoured two-storeyed wooden houses designed in the English Arts and Crafts manner of Charles Voysey and M. H. Baillie Scott.
On 29 April 1914, at the age of 37, Mair married Ethel Margaret Snow in Invercargill. Within months she contracted tuberculosis and died in September the following year, aged 33, leaving an eight-month-old son. Mair never remarried.
In 1918 he was engaged by the Defence Department as inspector of military hospitals, a position he held until 1920, when he became architect to the Department of Education. His public service career reached its pinnacle in 1923 when, in succession to John Campbell, he was appointed government architect. For the next 18 years Mair was responsible for most of the government buildings erected in New Zealand, many of them involving significant departures from tradition and precedent in style and construction methods. During his period of office modernist architectural precepts displaced the revivalist styles favoured for public buildings in the past; construction methods began to utilise concrete and structural steel instead of brick and timber.
During the difficult years of the depression, Mair saw to it that local architects and builders in towns outside Wellington were given work on government construction jobs. He was fastidious in his relationships with both superiors and subordinates and a tireless worker for the many committees on which he served.
Notable among the buildings built by the Public Works Department during his tenure as government architect are Rotorua's Blue Baths. These were designed in 1929 in Spanish mission style and had a lounge, tearooms and a colonnade between the two pools. Among his many other buildings were courthouses completed in Hamilton (1931), Ashburton (1938) and Blenheim (1939); and post offices in Napier (1930), High Street, Christchurch, and Cambridge Terrace, Wellington (1932), Tauranga (1938) and Lower Hutt (1943).
Mair's largest building was the Departmental Building in Stout Street, Wellington, which he designed in 1937. Modelled closely on a number of contemporary London office buildings, it was the largest office block in New Zealand at the time it was built. Strongly horizontal in emphasis, the stone exterior is unornamented and its long continuous bands of glazing and spandrel panelling sweep around the corner of Stout and Ballance Streets. It is a streamlined monolith with a floor area of more than five acres.
In 1940 Mair was elected a fellow of the Royal Institute of British Architects; after retiring in 1941 he received honorary life membership of the New Zealand Institute of Architects. He was for many years a member of the Town-planning Institute of New Zealand. At the age of 83, on 26 November 1959, John Mair took his own life at his home at Khandallah, Wellington. He was survived by his son, Lindsay, a Wellington architect.