John Campbell was born on 4 July 1857 in Glasgow, Scotland, to Janet McKechnie and her husband, Donald Campbell, a ship's chandler. He embarked on a career as an architect, serving his apprenticeship under John Gordon between 1872 and 1876 and working for him as an assistant draughtsman for a further three years. Gordon generally designed in an eclectic Greek Revival style, which was fashionable in Scotland. Campbell received a sound training but the buildings he designed in New Zealand reveal little of Gordon's influence.
Campbell arrived in Dunedin, New Zealand, in 1882 where he worked briefly for the firm of Mason and Wales. On 7 February the following year he was appointed to a temporary position in the Public Works Department in Dunedin. On 30 November 1888 Campbell was transferred to Wellington, where on 1 April 1889 he became draughtsman for the Public Buildings Department. That department merged with the Public Works Department in 1890, and Campbell's title became 'architect' in 1899. He remained in charge of the architectural design of government buildings in New Zealand until his retirement in 1922, holding the newly created title of government architect from 1909.
Campbell's first known work, an unbuilt design from the mid 1880s for the Dunedin railway station, reveals an interest in Baroque architectural elements which was to become his hallmark. However, the earliest buildings constructed to his design, such as the Porirua Lunatic Asylum (1891–94, now demolished) and the Dunedin police station (1895–98, modelled on New Scotland Yard, London), are predominantly Queen Anne in style.
Despite the completion of the Dunedin Law Courts in a Gothic style with a Scottish baronial inflection in 1902, Campbell successfully established Edwardian Baroque as the official architectural style for government buildings in New Zealand in the early twentieth century. In buildings as diverse in function as the Magistrate's Court, Wellington (1901–3), the Napier Government Buildings (1902–7, destroyed in the 1931 earthquake) and the Public Trust Office, Wellington (1905–9), Campbell used a limited range of Baroque design elements. His standardisation of the architecture of government buildings is nowhere more evident than in the design of post offices. During a post office building boom between about 1900 and 1914 he was largely responsible for two major post office buildings of similar design (the Auckland and Wellington chief post offices), and many smaller post office buildings.
Campbell developed two models for smaller post offices which, with some variation, were built throughout New Zealand. One consists of a two-storeyed block, with a hipped roof and a central gable with a porch on the front. The other consists of a block with a clock tower, situated on a corner site. Most of the towers of the extant buildings of this design no longer exist. However, the buildings with their rich Baroque decoration were the focal point of many towns and for many New Zealanders still represent the archetypal New Zealand post office building.
The destruction by fire of the timber portions of Parliament Buildings in 1907 provided Campbell with two of his largest commissions: the design of a new Government House (following appropriation of the existing Government House by Parliament) and new Parliament Buildings. Government House was built in timber in 1910 on Mt View, Wellington. In 1911 a competition was held for the design of the new Parliament Buildings. Believing, perhaps, that the commission should have fallen to him as government architect, Campbell decided to compete, although some architects in private practice submitted that government employees should not be allowed to enter a competition organised by their employer.
An attempt by the New Zealand Institute of Architects to boycott the competition in protest at this and other perceived irregularities was of only limited success. Thirty-three entries were received, two of which were prepared by Campbell, each in collaboration with one of his staff. A design by Campbell and Claude Ernest Paton won first prize, and a design by Campbell and Charles Lawrence fourth. A few months after the announcement, six members of the New Zealand Institute of Architects resigned their membership in protest. Campbell, an inaugural member of the Wellington branch of the institute and a fellow of the Royal Institute of British Architects since 1905, allowed his membership of the New Zealand institute to lapse following the competition.
The first stage of a revised design for Parliament Buildings, in the Edwardian Baroque style, incorporating elements of both the first- and fourth-prize plans, was under construction between 1912 and 1922. To Campbell's regret only the first stage of his design was built.
The style established for government buildings by Campbell was not adopted by his successors. However, he played an important role in establishing the office of government architect as a respected architectural practice in New Zealand. He was a quiet and unassuming man; his buildings are by contrast so ostentatious that they command attention. Although many have now been demolished, probably more examples of his work are known to New Zealanders, although anonymously, than buildings designed by any other architect.
Campbell travelled extensively after his retirement in 1922. He had been a keen yachtsman, and was active in the Presbyterian church. He was also an amateur landscape painter. On 18 April 1889 in Dunedin Campbell had married Mary Jane Marchbanks. It appears that they had no children. He died in Wellington on 4 August 1942, survived by his wife.