Thomas was educated at Royal Belfast Academical Institution and then entered Queen's College, Belfast. He had a glittering academic career but instead of continuing in the world of academia he entered the family business, which had been founded by his father and uncle, J & T Sinclair, who were provender merchants in Tomb Street. On the death of his father in 1867 he became head of the firm and he held that position until his own death in 1914.
In due course Thomas Sinclair was the man who penned the Ulster Covenant and at the first pre-covenant rally in Enniskillen he explained the document he had drafted. By this time Sinclair was an elderly man but he attended the eve of covenant rally in the Ulster Hall and the following day he signed the Covenant he had penned.