He was born at Rothesay on the Isle of Bute, Scotland, on 17 January 1828. He was the eldest of nine children of John Gillies, local lawyer and town clerk, and his wife, Isabella Lillie, daughter of a Glasgow businessman and granddaughter of a Huguenot refugee. Determined to train as a mechanical engineer, he was forced by his father to study law and trained in his father"s practice for four years.
John Gillies senior was so committed with various duties that his health suffered and after long discussions, it was agreed to emigrate to Otago, New Zealand. On 1 June 1852, Thomas Gillies married Catherine Douglas at Newcastle upon Tyne. They were soon joined in Otago by John Gillies Junior., who came over from Australia.
He then represented Mongonui 1870 (elected 30 March 1870.
Parliament dissolved 30 December 1870) then Auckland West 1871 to 1875 (resigned). Two electorates in the North Island.
He was the seventh Superintendent of Auckland Province from 1869 to 1873. He was a cabinet minister, and held the positions of Attorney-General (August 1862) in the Domett Ministry, Postmaster-General and Secretary for Crown Lands (1863–1864) in the Whitaker–Fox Ministry, and Colonial Treasurer (1872) in the third Stafford Ministry.
Gillies joined the practise of his father John Gillies and John Hyde Harris in July 1857.
In the 1860s, he ran a law practice in Dunedin with William Richmond, a fellow (ex) Member of Parliament.
He was the Member of Parliament for Dunedin Country from 1860 (after a by-election), then Bruce 1861 to 1865 (resigned). Two electorates in the South Island.
