Thomas William Kirk was born in Coventry, Warwickshire, England, on 30 June 1856, one of nine children of Sarah Jane Mattocks and her husband, Thomas Kirk, a timber merchant's clerk. His family emigrated to Auckland, New Zealand, on the Gertrude in 1863. Kirk attended St James' School, then Auckland Grammar School. Both he and his brother Harry, the first professor of biology at Victoria College, Wellington, inherited an interest in science from their father, who taught natural science at Wellington College from 1874.
That year Kirk joined the civil service as an assistant with the Geological Survey Department. He served there under James Hector and became recognised as a member of the small New Zealand scientific community. During this period he contributed articles on biological subjects to numerous publications including the Transactions and Proceedings of the New Zealand Institute, Nature, and a French journal of conchology. He was elected a member of the New Zealand Institute in 1878, the Geological Society of Australasia in 1887, the Royal Microscopical Society in 1889 and the Linnean Society of London in 1890. He married Edith Dixon Callcott on 13 December 1883 at Wellington.
In 1892 Kirk became acting biologist of the newly formed Department of Agriculture; by 1895 he was biologist and chief of the division of biology and pomology. His primary interest was in the development of a viable fruit-growing and exporting industry, which was dependent on the support given by the government through the department. In 1892 Kirk initiated the publication of leaflets to fruit-growers to educate them on propagation techniques and to help them identify disease. Fruit tree diseases, particularly codlin moth and scale, were ravaging orchards. His division consulted widely with orchardists and with their support promoted state action against plant pests and diseases.
In 1896 the Orchards and Garden Pests Act, for which Kirk had been agitating, was passed; it instituted procedures to guard against imported diseases. Inspectors were given considerable powers to destroy or fumigate plant material, or to send it back to its country of origin. This legislation was strengthened in 1903 when the inspectors gained the power to control pests within New Zealand. As a result many older, abandoned orchards that were harbouring disease were uprooted by departmental inspectors. Kirk also initiated the biological control of pests when he introduced the Australian ladybird (vedalia) to control the cottony cushion scale.
Kirk's work, combined with developments in cool storage and a system of incentives to successful exporters, meant that fruit could be successfully exported by the end of the first decade of the century. By 1912 there was a significant trade with South America and regular shipments to the United Kingdom. Kirk promoted the planting of apple and pear trees, particularly on the Moutere hills in Nelson where he eventually invested in land himself. He also supported the development of co-operative grower organisations and provided, through the Department of Agriculture, organisational support to the New Zealand Fruitgrowers' Federation from its formation in 1916. Government encouragements to export and cheap loans to prospective orchardists led to a speculative boom and overplanting. It was 20 years before the acreage of orchards stabilised at a much lower level, although a viable export industry had been established.
Kirk retired in June 1921 at the height of the orcharding boom. He continued his interest in botanical and scientific organisations, but increasingly devoted himself to the affairs of the Masonic order. He was a past master of Coronation Lodge, a foundation member and first master of Lodge Tawera-o-Kapiti and past senior grand deacon of the Grand Lodge of New Zealand. He was known for his lively sense of humour, and was a noted raconteur. He died on 19 May 1936 at Raumati Beach, survived by his wife and a son. Kirk's scientific knowledge and administrative skills had been instrumental in establishing a viable fruit export industry in New Zealand.