Later life
In 1660 at the Restoration, Ossory accompanied Charles II back to England. That same year he was appointed Lord of the Bedchamber to Charles II, a post he held until his death. Emilia was naturalised as English by act of Parliament.
In 1661 Ossory became an MP for Bristol in the English and for Dublin University in the Irish house of commons.
In 1662 Ossory was called to the Irish House of Lords under a writ of acceleration as Earl of Ossory. His father had held the title "5th Earl of Ossory" as one of his subsidiary titles. The acceleration made Thomas Butler the 6th Earl of Ossory.
In 1665 Ossory was appointed lieutenant-general in the Irish army. 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.
He was created an English peer as Baron Butler of Moore Park by being summoned to the English House of Lords by a writ on 17 September 1666. Almost as soon as he appeared in the House of Lords, he was imprisoned for two days for challenging the Duke of Buckingham.
He acted as deputy for his father, 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, starting the Third Anglo-Dutch War (1672–1674) in an action that he is said to have greatly regretted later in life.
In May 1672 he fought against the same enemies in the Battle of Solebay, serving with great distinction on both occasions.
While visiting France in 1672, he rejected the liberal offers made by Louis XIV to induce him to enter the service of France.
In August 1673 he added to his high reputation by his conduct during the Battle of Texel in August 1673. From 1677 until 1679, he served alongside his father as a Lord of the Admiralty.
Ossory was intimate with William, Prince of Orange, and in 1677 he joined the allied army in the Netherlands, commanding the British contingent and excelling at the siege of Mons in 1678.
In 1680 he was appointed governor of English Tangier, but his death prevented him from taking up his new duties.