Thomas Arthur McCormack was born on 27 April 1883 at Napier, the son of Irish parents Elizabeth Kirwin and her husband, Thomas McCormack, a labourer. At the Marist brothers' school in Napier he was interested only in athletics and drawing. He became captain of the school's rugby and cricket teams and, with his friend Lewis Evans, he experimented with watercolour landscapes when they roamed the local wetlands and beaches. From an early age he was a keen observer of the natural world, its movement, form and colour. After leaving school he had lessons in black-and-white drawing from R. N. Anderson, an English landscape artist who taught at Napier Technical School.
At 17 McCormack suffered a severe illness that left him an invalid for nearly three years and drastically curtailed his physical activity for some time. From about 20 he turned his attention almost entirely towards painting. While strengthening his own handling of the medium of watercolour, he taught painting and drawing at intervals in Masterton.
In 1921 he moved to Wellington, where on 17 March 1924 he married Mabel Gertrude Craddock. There seem to have been no children of the marriage. During the depression, when Thomas sold no paintings, the couple relied on Mabel's earnings, either as a music teacher or as a nurse. They rented a studio flat in Hill Street, Thorndon, which had belonged to the artist D. K. Richmond. McCormack continued to study painting through reading in the Wellington Public Library and contact with other painters, especially his friend, Nugent Welch. He also exhibited regularly with the New Zealand Academy of Fine Arts. His knowledge of European art was expanded when he left New Zealand for the first and only time in 1928 to spend eight months in Australia.
McCormack would show work on request to patrons in his studio, which faced south and was lit through long sash windows. His framed paintings were stored under the windows behind green cloth curtains and he would cautiously bring them out one at a time to place on an easel. If he found the viewer unresponsive and uninformed, he would not continue.
His reputation as a painter continued to grow during the 1930s and 1940s and his calligraphic brush stroke increasingly reflected Chinese evocation rather than Western representation. The first serious reviews of his work appeared in Art in New Zealand in 1936: the artist Roland Hipkins commented that 'His efforts…have remarkable freshness, breadth and simplicity, with spontaneous brush work and a rare quality of colour', and that 'he has sensed the spirit of his native country…with such beauty and creative passion that, I believe, his art will remain'. In the same issue J. C. Beaglehole compared McCormack's sincerity and seriousness with Cézanne's: 'the same devotion to the technical problem of the realisation of the idea'.
In 1945 a solo exhibition of his work, sponsored by the Department of External Affairs, toured to New York and Ottawa. Ten years later Helen Hitchings included him in a New Zealand show in London and C. Millan Thompson organised his first solo exhibition in New Zealand at the Architectural Centre gallery, Wellington. McCormack was one of five New Zealand watercolourists shown by the Auckland City Art Gallery in 1959, and in the same year a joint retrospective with Nugent Welch was held at the New Zealand Academy of Fine Arts in Wellington. The academy also exhibited a solo show of 170 works in 1971–72.
McCormack was made an OBE in 1956 for his services to art. His wife, Mabel, died in 1959, and he returned to Hawke's Bay in 1965. He lived there for the remainder of his life at the Little Sisters of the Poor Home in Hastings, where he died, aged 90, on 26 June 1973.
Essentially a self-taught artist, McCormack believed that 'Art is a matter of feeling and expression'. His painting was never static, and over 60 years he developed its subtle poetry in landscapes, still-lifes, figure studies and a few portraits. His exquisite control of the medium of watercolour is equalled in New Zealand only by Frances Hodgkins and Rita Angus. Although his best late work is privately owned, examples can be seen in many public collections. The strongest holdings are in the Hawke's Bay, Dunedin and Auckland art galleries, and in the Museum of New Zealand Te Papa Tongarewa, Wellington.