When he succeeded to the title, Thomas Petre was a 52-year-old bachelor, but the need for an heir soon pushed him into marriage. Before the year was out he had arranged to marry Mary Clifton, daughter of the future Jacobite Sir Thomas Clifton, who brought with her a dowry of £5,000. Along with the title, he also inherited the Petre estates, together with the debt trust created by his brother, William Petre, 4th Baron Petre. The debt trust was still in existence in 1697 when the 5th baron composed a will, but by 1712 the estate was said to be free of encumbrances.
A committed Catholic who not only maintained his own chaplains but also provided a base for missionary Jesuit priests and held land on behalf of a Catholic monastery in France, Petre was disabled from sitting in the House of Lords by the provisions of the 1678 Test Act.
At the Essex election of 1685, Sir John Bramston‡ described how Petre joined the cavalcade of court supporters near Chelmsford with about 300 ‘gentlemen, his kinsmen, his tenants, and other freeholders, his neighbours …’.
James II expected Petre to play a key role in the forthcoming parliamentary elections and he did his best to live up to expectation: ‘The Lord Petre, as lord lieutenant, by the king’s command visited the country, as well as the corporations. He carried divers gentlemen, papists, with him in his circuit …’.
In September 1688, Robert Spencer, 2nd earl of Sunderland sent him ‘a list of gentlemen intending to stand with his majesty’s approbation for Members of Parliament within your lieutenancy’ and directed him ‘to give them all the assistance and countenance you can’.
The Revolution of 1688 marked the end of Petre’s political career, though his devotion to James did not extend to following him into exile. There are some indications that in the aftermath of the accession of William and Mary he became a target for those who resented his earlier support for James II, although the reference in the Calendar of State Papers, Domestic to his imprisonment in the Tower in 1689 appears to be erroneous. It is probably a mistranscription for Henry Mordaunt, 2nd earl of Peterborough.
At his death on 5 Jan. 1707 Petre was succeeded by his son, Robert Petre, 7th Baron Petre. His second son, Thomas, had died an infant in 1691. He made provision for a portion of £5,000 for his daughter, Mary, but she died unmarried in April 1713, probably (like her brother) of smallpox.
