Thomas William Downes was born on 18 February 1868 at Wellington, New Zealand. He was the son of Emily Adele Harrington and her husband, Thomas William Downes, a surveyor who had earlier been a drawing teacher in Nelson. The Downes family moved to the Rangitīkei township of Bulls in 1874 or 1875. Downes attended school there and later worked as a cabinetmaker. He married Margaret Wilhelmina Naismith Thomson on 26 May 1890 at Bulls.
In 1898 Downes shifted to Whanganui and set up in business on his own account, initially as a cabinetmaker and later as a grocer and an indent agent. From May 1919 to July 1920 he operated a large commercial motor canoe on the Whanganui River. On 1 January 1921 the Wanganui River Trust appointed him supervisor of river works and ranger for the domain lands within its jurisdiction. His salary was £100 per annum. The trust had been empowered by an act of Parliament in 1891 to open up or improve the navigation of the Whanganui River. It had started well but heavy floods along with the shortages of revenue, labour and material during the First World War had resulted in a general decline in its work. The situation improved during the 1920s but in the early 1930s the government subsidy for river maintenance was progressively reduced, and ceased in January 1934, leaving the trust with only a supervisory role.
On the trust's behalf, from 1934 Downes oversaw work carried out by the owners of the river steamers. It is said that he knew the full length of the Whanganui River better than any other European. He lived at Whanganui East, and used his private launch, the Tangāhoe, to travel on the river. He had his own well-appointed hut on the right bank opposite Atene and developed it as his headquarters. It is now part of the hut system on the Whanganui and known as Downes Hut.
From the time he lived in Bulls, Downes had been interested in local history. One of his first articles, published in 1910 in the Transactions and Proceedings of the New Zealand Institute, was on the 'Early history of the Rangitīkei and notes on the Ngāti Apa'. Throughout the years he worked for the trust he took every opportunity to research and record all aspects of local history from his many Māori friends. This interest led him to become a member of the Polynesian Society in 1910. He was elected a member of the executive council in 1931 and was a frequent contributor to the society's journal, as well as to the Transactions.
Downes documented the early history of the Whanganui district in his book Old Whanganui, published in 1915. This was followed in 1921 by his History of and guide to the Wanganui River. A later manuscript, 'River ripplets', was published in 1993. These books have become increasingly valuable resources on the Māori history of the Whanganui River. Some of the material compiled by Downes had been recorded earlier by G. F. Allen and the Reverend Richard Taylor, but most was based on original research. Downes lived at a time when much Māori traditional history was available orally and, along with other self-taught amateur historians, he took it upon himself to record as much information as he could, feeling it would otherwise be lost for all time.
Downes was also a watercolourist and sketcher, an interest he inherited from his father. A selection of his watercolours, to illustrate 'River ripplets', is held at the Alexander Turnbull Library, in Wellington. (His artwork and that of his father are often confused because both have the same name.) When the fifth Science Congress of the Royal Society of New Zealand was held in Dunedin in 1935, Downes was the president of the anthropology section. At the time of his death he was working with L. J. B. Chapple as co-author of the centennial history of Whanganui. The book, Wanganui, was later completed with H. C. Veitch replacing him as co-author.
Thomas Downes was a modest, quiet and unassuming man who took a deep interest in everything associated with the Whanganui River and with Māori history and tradition. He had no personal collection of river material; it was all given to the Wanganui Public Museum or to the Polynesian Society and its officers. He was still employed as the supervisor of the Wanganui River Trust when he died at his home in Whanganui on 6 August 1938. He was survived by his wife and a daughter.