He served as Prime Serjeant and as a judge of the Court of Common Pleas.
He sat in the Irish House of Commons as member for Dunleer for many years.
The bishop was a cousin of, and on friendly terms with, the earlier Thomas Tenison who was Archbishop of Canterbury.
His father was a wealthy landowner of scholarly tastes, who was a member of the Dublin intellectual circle which revolved around Esther Johnson, the beloved Stella of Jonathan Swift; Henry and Swift had been friends at Trinity College Dublin.
Thomas and his three sisters were raised by relatives, most likely by their uncle Richard Tenison junior, MP for Dunleer, who founded the County Monaghan branch of the family.