[2] Thomas's mother was a second cousin once removed of his father as she was a granddaughter of Black Tom, the 10th Earl of Ormond.

As the eldest living son, he was the heir apparent and was styled with the corresponding courtesy title, which initially was Viscount Thurles but changed to Earl of Ossory when his father became marquess in 1642.

In 1652 when Cromwell had completed the conquest of Ireland, his mother brought Ossory and his sibling to London where she obtained a pension of £2000 per year from the income from her Irish estates under the condition that none of that money would be passed on to her husband.

Thomas and Emilia had eleven children,[13] including two sons:[14] —and three daughters: In 1660 at the Restoration, Ossory accompanied Charles II back to England.

In 1665 during the Second Anglo-Dutch War (1665–1667), a fortunate accident allowed Ossory to take part in the Battle of Lowestoft against the Dutch.

[10] He acted as deputy for his father,[27] who was lord-lieutenant of Ireland, and in parliament he defended Ormond's Irish administration with great vigour.

On 12 March 1672 he attacked the Dutch Smyrna fleet with HMS Resolution,[28] starting the Third Anglo-Dutch War (1672–1674) in an action that he is said to have greatly regretted later in life.

Ossory was intimate with William, Prince of Orange,[30] and in 1677 he joined the allied army in the Netherlands, commanding the British contingent and excelling at the siege of Mons in 1678.

[34] Some say Ossory's body was later taken to Ireland and reburied in the family vault in St Canice's Cathedral, Kilkenny.