The bust of the figure is depicted at three-quarter length, turning towards the right; clothed in a dress coat and waistcoat of blue velvet, lined with white fur, on top of that a white ruff embellished with lace similar to that on the cuffs; his hat is held under his left arm.

Anthony Clark, in 1985, was unsure of the exact location of the painting following its appearance on the New York market at the beginning of the 1900s, erroneously attributed to George Romney.

The fame attained by Batoni amongst the travellers, especially the English, who arrived in Rome during the second half of the 18th century is undisputed. The notoriety of the painter in England is evident by the enormous number of portraits (almost two hundred) in existence depicting important English and Irish characters. These works cover a time frame of almost forty years, from the mid-1740s to the mid-1780s and constitute, in the middle two decades, the majority of Batoni’s production.

The sitter for the present portrait is Sir Thomas Thornhill (ca. 1735 - 1800), who passed through Rome on the Grand Tour. The Thornhills were descended from an ancient family of Anglo-Danish landholders in Yorkshire. Much of their holding was in the Calder Valley between Pontefract and Dewsbury. The family fought with Harold at the Battle of Stamford Bridge 1066 and their ancestor Gamall is mentioned in the Domesday Book 1086. The Thornhills settled and established themselves near to Dewsbury at Thornhill Lees. They also held the sub manor of Midgley, hence the similarity between the Arms of Midgley and Thornhill. Thomas succeeded to his uncle's estates at Fixby, Yorkshire and in 1779 married Eleanor Lynne. Prior to this, c. 1761-4, Thomas undertook a European tour in the company of his younger brother, George, who had a house at Diddington. Thomas is recorded at Capua on 8 February and 12 March 1761 travelling with the Swiss apothecary, intellectual and poet, Jean-Baptiste Tollot acting as their guide (Cf. J. Ingamells, A Dictionary of British and Irish Travellers in Italy 1701-1800, New Haven, 1997, p. 938). At Paris, they made the acquaintance of and befriended, Laurence Sterne. A final reference records the purchase of four paintings from the marine painter, Joseph Vernet by 'M. Thornhill, cadet' on 17 May 1764 and a further two more in 1766 (Léon Lagrange, Joseph Vernet et la Peinture au XVIII siécle, Paris, 1864, pp. 343-46).

Fixby Hall was modernised by Thomas Thornhill in the mid-18th century and as part of these renovations, he built the Orangery as a wedding present to his bride Eleanor Lynne. Thomas's eldest son, also called Thomas, inherited the estate and notably employed Richard Oastler, the social reformer as Steward to the estate. Thomas's sister, Clara, eventually inherited Fixby and also bought Rushton Hall in Northamptonshire. She became a personal friend of Charles Dickens and it is thought that he gained his inspiration for Haversham Hall in Great Expectations from his visits. The Thornhill family lived full time at Fixby Hall until 1809 when they moved to Norfolk and the building was divided into three livings.

The present portrait is a fine example of Batoni's Grand Tour portraiture and reveals, in the anatomical precision with which he delineates the features of the sitter, the crisp articulation of his hands, and the fresh and assured handling of paint, why he was the portrait painter in Rome.

Clark dates the work around 1764-65, comparing it stylistically with Portrait of David Garrick, Ashmolean Museum, Oxford, of 1764, and Portrait of Marchese Manuel de Roda y Arrieta, Reale Accademia di Belle Arti di San Fernando, Madrid, of 1765.