James McCosh Clark was born in Beith, Ayrshire, Scotland, on 12 August 1833, the son of Archibald Clark, a merchant, and his first wife, Margaret McCosh, whose father was a wealthy coal mine owner. James arrived in New Zealand with his father and stepmother (Archibald Clark's third wife) in 1849. The next year his father started a clothing manufacture and wholesale business, in which James became a partner in 1856 or 1857. He joined the volunteers and rose to captain, commanding the No 6 Company of the Auckland Volunteer Rifles in Waikato in 1863. He became active in church work serving as treasurer of the general assembly of the Presbyterian Church of New Zealand from 1862 to 1872.
For nearly two decades James McCosh Clark was prominent in the civic and commercial life of Auckland. In the late 1860s, when the war in Waikato had depressed economic activity in the Auckland province, he encouraged the local flax industry, which was to provide much-needed employment. In 1870 he was a member of the Auckland Provincial Council for Newton. He became wealthy through his investments in Thames goldmining companies, particularly the Moanataiari, of which he was a founder and for some years a director and chairman. When his father died in 1875 he became the senior partner in the family business, Archibald Clark and Sons.
On 8 April that year he married Kate Emma Woolnough in Melbourne, Australia. She was the daughter of Susan Bonner and her husband, Henry Woolnough, an architect, and was born at Ipswich, Suffolk, England, on 15 May 1847. Of Kate Woolnough's early life and education little is known. In London she had earned a living doing research for writers, often in the British Museum. After her marriage she and James made their home in Auckland, where their five children were born.
James McCosh Clark stood for Parliament, unsuccessfully, in 1875, 1880 and 1882, and was a strong opponent of the popular Sir George Grey. He associated with a small group of influential men, as a director of the New Zealand Insurance Company, the Auckland Fibre Manufacturing Company and the Bank of New Zealand. From 1879 to 1881 he was president of the Auckland Chamber of Commerce and in 1880 was a member of a royal commission investigating the country's railways. He published in 1879 an influential analysis of New Zealand's finances, arguing that an inadequate share of government loan expenditure was allocated to Auckland.
McCosh Clark was also active in local body administration. He was a member of the Auckland Harbour Board and an Auckland improvement commissioner, and chaired the education board. In 1880 he was a popular choice for mayor, a position he held for three years. He took a keen interest in the development of the infrastructure and cultural assets of the city. The laying of the original horse-drawn tramways and the purchase of a site for the town hall were initiated during his term, and the first site of the public library (now the art gallery) chosen. He spent most of his honorarium on books for the library. It was also during his mayoralty that the city began to expand its boundaries. He proposed an ambitious plan, but settled for the inclusion of Ponsonby, Karangahape and Grafton, making Auckland the largest municipal area in the country and increasing its commercial status and political influence. A more controversial action was bringing into operation the Contagious Diseases Act 1869 with the establishment of a contagious diseases hospital in 1883.
In 1865 James McCosh Clark had bought a large Remuera home, The Tower, on 12 acres of land. Kate Clark, with the support of domestic servants, expertly performed her role as a society leader and mayoress at The Tower, considered to be 'the best appointed establishment in or near Auckland'. Described as an excellent musician, 'an accomplished painter and a woman of rare good sense', she was a generous patron of music and art. At her home she organised groups of women for painting, reading and dramatic activities. She and her husband helped to reform the Auckland Society of Artists as the Auckland Society of Arts in 1880. Kate was on the society's committee for 10 years, and exhibited oils, watercolours and drawings; James was president from 1881 to 1888. They made generous donations of pictures to the Auckland Art Gallery when it opened in 1888.
Kate Clark was also an important figure in charitable organisations in the city. In 1882 she worked closely with Eliza Cowie, wife of the Anglican bishop of Auckland, to establish the Girls' Friendly Society, of which she was vice president. She was on the management committee of the Women's Home in Parnell, for which her principal role was that of publiciser and fund-raiser, and she was involved in the Jubilee Institute for the Blind. Also keen on physical recreation and the outdoors, she went on climbing expeditions, and was active in the Auckland Skating Club.
The family firm continued to prosper in the 1880s but James McCosh Clark's other commercial ventures were less successful. Like many other business leaders he was infected by the speculative mentality engendered by gold discoveries: encouraged by the economic boom of the early 1880s, he borrowed heavily to invest in several ambitious companies. The Te Aroha Battery Company, which he established with Josiah Firth, yielded little gold and was sold in 1887 at great loss. In December 1881 he was one of a group which set up the Thames Valley and Rotorua Railway Company to open up land acquired from Māori. With Thomas Russell and others he was involved in the New Zealand Native Land Settlement Company, formed to sell land they had acquired in Waikato to British investors and settlers; but as the economy declined, few sales were made. At the height of his popularity as mayor, McCosh Clark was described as 'one of our nouveaux riches'. In 1889 there was little esteem or sympathy for him when, after several business failures, he left with Kate for England.
In England Kate Clark pursued a career as a writer: she had completed her first book before her husband's financial ruin forced them to leave New Zealand. A southern cross fairy tale, which she partly illustrated, was published in London in 1891. A Christmas story for children, it attempted to translate northern hemisphere Christmas folklore to New Zealand while instructing children about the colony's natural features and native creatures. Notes by Andreas Reischek on birds and A. P. W. Thomas on geological features were included. In 1894 she published a collection of verse, Persephone and other poems. Her last work, Māori tales and legends (1896), was intended to interest and instruct young people about New Zealand and the Māori. Although its style was sentimental and dramatic, its detailed, scholarly notes – referring to such sources as Edward Tregear, John White, F. D. Fenton, George Grey and Tāwhiao, the Māori King – showed the depth of her research. In addition to her books she also wrote newspaper articles and short stories for magazines.
In 1899 Kate Clark and Beatrice Webb, who had recently visited New Zealand, represented the National Council of Women of New Zealand at the London meeting of the International Council of Women. James McCosh Clark had died in Hastings, Sussex, on 26 January 1898, and Kate returned to New Zealand in 1900. Little is known of her life after this time. She died at Auckland on 30 November 1926, survived by her two sons. The McCosh Clarks' former home, The Tower, had been sold in 1895 to become part of King's College (now King's School).