Thomas Thynne, 1st Viscount Weymouth (1640 – 28 July 1714) was an English politician who served as president of the Board of Trade from 1702 to 1705.

He was created 1st Viscount Weymouth, on 11 December 1682, with a special remainder; if he lacked male heirs among his own descendants, the title would be inherited by his two brothers, James and Henry Frederick.

On 13 December 1688, Weymouth carried an invitation to William III, Prince of Orange at Henley-on-Thames, along with Thomas Herbert, 8th Earl of Pembroke, after the flight of King James II in the Glorious Revolution.

William Legge, 1st Earl of Dartmouth wrote that "Lord Weymouth was a weak, proud man, with a vast estate...

He was very liberal to non-jurors, though he always took the oaths himself; which occasioned his house being constantly full of people of that sort, who cried him up for a very religious man; which pleased him extremely, having affected to be thought so all his life; which the companions of his youth would by no means allow."

Adrian Gaunt, Alan Maynard, Robert Smythson, the Earl of Hertford and Humpfrey Lovell all contributed to the new building but most of the design was Sir John's work.

Formal gardens, canals, fountains and parterres were created by George London with sculptures by Arnold Quellin and Chevalier David.

He employed George London to lay out a vast complex of ornate terraced flower beds, with symmetrical paths and avenues, to furnish Longleat with a decorative environment, which stretched for the most part eastwards, across the leat (having diverted 'the long lete' with a canal), and on up into what is now the safari park.

Thomas Ken, Bishop of Bath and Wells, when deprived of his see by William and Mary in 1691 after he refused to transfer his oath of allegiance from James, on the grounds that once given, it could not be forsworn, was given lodgings at Longleat and an £80 annuity by the 1st Viscount Weymouth, a friend since Oxford days.

Taking up residence on the top floor at Longleat for a period of some twenty years, he exerted a profound influence upon Thomas, becoming what some might describe as his conscience.

Not that its interior ever matched the architectural finery of equivalent chapels in other stately homes, but it was in any case evidence of the devout spirit which prevailed at Longleat over that particular historical period.

While living in the house, Bishop Ken wrote many of his famous hymns, including 'Awake my soul', and, when he died in 1711, bequeathed his extensive library to the 1st Viscount.

In his "Longleat: the Story of an English Country House" (London, 1978), David Burnett records (somewhat improbably, but on the evidence of the Bath estate archive): '...

In 1694 a Polish baron had written to Thomas Thynne, 1st Viscount Weymouth asking if he could lease 4,000 acres (16 km2) and the Irish estate town of Carrickmacross in order to settle 200 Protestant families from Silesia.

Thomas consented, but the agreement was cancelled when the baron announced his intention to demolish the town and rebuild it in the Polish style."

The stern language of its ninth statute stated: "The master shall make diligent enquiry after such as shall break, cut or deface or anywise abuse the desks, forms, walls or windows of this school, and shall always inflict open punishment on all such offenders".

The 1st Viscount Weymouth died in 1714, without surviving male issue, and bequeathed his estates to his grand-nephew, also named Thomas Thynne, and ancestor of the Marquesses of Bath.

It was not until 1826 that Robert's grandson, Evelyn John Shirley, laid the foundations of a mansion worthy of the family and estate near the banks of Lough Fea.