Thomas Millar was born on 11 August 1879 in Carrickfergus, Co Antrim, Ireland (modern-day Northern Ireland), a market town on Belfast Lough which is dominated by the Norman fort, Carrickfergus Castle. He was the son of Thomas Millar and Agnes Magawan and grew up in a Church of Ireland household.
He served his apprenticeship with Harland & Wolff and subsequently worked for Vickers of Barrow, Workman, Clark & Co. of Belfast and Harland & Wolff of Belfast.
Thomas was married in Belfast in the second half of 1900 to Jane "Jennie" Ruddick (b. 1879 in Belfast, daughter of William Ruddick, an artisan). The couple would have two sons: Thomas (b. 1901) and William Ruddick (b. 1907).
Thomas is absent from the 1901 census of Ireland but his wife and first son are listed at 10 Meadow Street in the Dock Ward of Belfast City, the home of his wife's widowed father and several of her siblings. By the time of the 1911 census Thomas and his family were resident at 14 Hillman Street, also in the Dock Ward, and he was described as a fitter. He reportedly helped build the engines for both Olympic and Titanic.
Thomas was widowed on 18 January 1912 when his wife Jane died aged 32. He decided to go to sea shortly after, leaving his sons with an aunt. Before he left on Titanic he gave each of his sons two new pennies which are still in the possession of current day family.
When he signed on to the Titanic, in Southampton, on 6 April 1912, he gave his address as 19 Meadow Street, Belfast. His previous ship had been the Gothland. As assistant deck engineer he could expect monthly wages of £9, 10s. The Titanic was reportedly his second sea voyage.
Thomas Millar died in the sinking. His body, if recovered, was never identified.