James Armour Johnstone was born in Glasgow, Scotland, on 25 June 1859, the son of a Congregational minister, David Johnstone, and his wife, Jessie Baird Thomson. In 1868 the Johnstones emigrated with their four children to Dunedin, New Zealand, where David Johnstone became minister of the Leith Street church.
James attended the High School of Otago from 1873 to 1875, and then joined the stock and station agency Wright Stephenson and Company. A strong-willed and high-spirited teenager, he did not take easily to the routine life of an office boy. He ran off home one day but his mother promptly sent him back. Happily, he soon found his way into the more practical side of the business, and in this work received wide training. For 10 years he worked throughout Otago and with every class of livestock. He developed a devotion to the firm, an integrity of character, an industrious lifestyle and a remarkable acumen and decisiveness in business. In 1885, when still in his mid-20s, he was made a partner in the firm.
On 31 May 1894, at Dunedin, James Johnstone married Margaret Forrest Donald, a governess to a family at West Taieri. They made their home at Maori Hill, and were to have three daughters. In 1899, on the retirement of John Wright and John Stephenson, James Johnstone became senior partner in the stock and station agency. When the firm was formed into a public company in 1906 he was made chairman of directors. Ill health forced him to relinquish this position, but he remained a managing director until his death. It was said that as the firm grew it 'reflected vividly' his personality and character. Yet it was less in his career as a company director than as a stock breeder that Johnstone played a major role in New Zealand's agricultural development.
A world tour with his family in 1907 was the first of many trips abroad, mostly to buy or sell stock or to exhibit or judge in agricultural shows, especially in Australia and Argentina. In 1910, in partnership with W. D. Hunt, Johnstone purchased 900 acres of the seaward side of the Bushey Park estate, near Palmerston, to which they appointed Alex Twaddle as manager. When the whole of the estate came on the market in 1920 Twaddle persuaded Johnstone to become the sole owner.
Aware of a developing demand in South America for Corriedale sheep, a breed pioneered in New Zealand, Johnstone in 1920 bought the entire stud flock of Leonard White of Rakaia, and a further 200 ewes from James Ross. This was a landmark purchase, with a reputedly record price of £10,000 for just under 1,000 sheep. Bushey Park became a famous stud-stock breeding centre, sending Corriedales to many countries.
Johnstone set about making the estate a show-place, erecting new buildings and planting thousands of trees. From Scotland he imported shorthorn cattle, whose progeny were prominent in shows in New Zealand and had an influence on the country's beef breeds. A Clydesdale stud kept farm teams up to strength. Johnstone was a founder and the first president of the Clydesdale Horse Society of New Zealand.
James Johnstone played a leading role in a number of agricultural and commercial organisations, including the Shorthorn Society of New Zealand, the Royal Agricultural Society of New Zealand, the New Zealand Refrigerating Company and the Commercial Union Assurance Company. He was also vice consul of the Argentine Republic in Dunedin. He was known to be a perfectionist and an obsessive worker, who would leave home at 7 a.m. and return late in the evening. His success as a businessman and stock breeder made him a wealthy man. On weekends and school holidays the family would travel out to Bushey Park from Dunedin in a chauffeur-driven Daimler. Margaret Johnstone and her daughters entertained expertly. The governor general, Lord Bledisloe, visited the farm on one occasion.
Margaret Johnstone was for many years president of the Royal New Zealand Society for the Health of Women and Children (Plunket Society), while her husband was financial adviser to the society and a member of its council, and founder-director of the Karitane Products Society. The Johnstones were close friends of the Plunket Society's founder, Truby King.
In his 70s James Johnstone had a bad fall from his horse, from which he never fully recovered. He died at home in Dunedin on 8 May 1933. Margaret Johnstone died just over three months later, on 20 August. To further the development of farming in New Zealand, James Johnstone had instituted a trust in Wright Stephenson to provide grants for research and study in wool-classing, seed and fertiliser specialisations, accounting and promotion. After his death his daughter Ethel Johnstone carried this on, and also endowed a J. A. Johnstone Memorial Laboratory at Lincoln College for research relating to livestock.