On the site of a former Augustinian priory, Longleat is the earliest of what Sir John Summerson called the Elizabethan prodigy houses, but the exact date of its construction is not known. The estate had been purchased by Sir John Thynne in 1541, and the house converted for domestic use, although this was lost by fire in 1567. The Elizabethan house is datable to after that year, possibly to designs by Robert Smythson (1535-1614), but it was incomplete on Sir John's death in 1580.

In 1751 Longleat was inherited by Thomas Thynne (1734-96), the eldest son of the 2nd Viscount Weymouth, who succeeded as the 3rd Viscount. He served as Lord of the Bedchamber to George III in 1760-63; Master of the Horse in 1763; Lord Lieutenant of Ireland in 1765; Groom of the Stole in 1775 and 1782-96; Secretary of State for the Northern Department in 1768, and Secretary of State for the Southern Department in 1768-70 and 1775-79. In 1778 he was created a Knight of the Garter, and in 1789 he was created 1st Marquess of Bath.

In 1759 Weymouth married Lady Elizabeth Cavendish-Bentinck (d1825), the eldest daughter of the 2nd Duke of Portland. Elizabeth's brother, the 3rd Duke of Portland, was Robert Adam's patron for the unexecuted designs for Portland House, New Cavendish Street, London, in c1770. It was doubtless through this family connection that Adam came to Weymouth's attention, and in 1775 he was commissioned to design a new wing for Longleat, and a scheme to convert the gallery into a library. These works were not executed.

Substantial alterations were made to the fabric and interior of the house by the 2nd Marquess of Bath in 1806-13, to designs by Sir Jeffry Wyatville (1766-1840), and in the 1870s the interiors were redecorated by the 4th Marquess, to designs by John Dibblee Crace (1838-1919). Longleat remains the home of the Marquesses of Bath, and is open to the public.

Literature:
A.T. Bolton, The architecture of Robert and James Adam, 1922, Volume II, Index pp. 21, 62; N. Pevsner, and B. Cherry, The buildings of England: Wiltshire, 1975, pp. 308-9; D. King, The complete works of Robert & James Adam and unbuilt Adam, 2001, Volume II, p. 131; 'Longleat Park', British listed buildings online; 'Thomas Thynne, 1st Marquess of Bath KG (1734-1796)', Oxford dictionary of national biography online

Frances Sands, 2013