Thomas Sandby RA (1721 – 25 June 1798) was an English draughtsman, watercolour artist, architect and teacher. In 1743 he was appointed private secretary to the Duke of Cumberland, who later appointed him Deputy Ranger of Windsor Great Park, where he was responsible for considerable landscaping work.
Along with his younger brother Paul, he was one of the founding members of the Royal Academy in 1768, and was its first professor of architecture. His most notable architectural work was the Freemason's Hall in London (now demolished).
Sandby was born in Nottingham, the son of Thomas Sandby, a textile worker, and was self-taught as a draughtsman and architect. Paul Sandby was his brother.
According to the autobiography of the architect James Gandon, Thomas and his brother Paul ran a drawing academy in Nottingham before they went to London in 1741, to take up employment in the military drawing department at the Tower of London (a post procured for them by John Plumptre, MP for Nottingham). Another source says that Thomas initially went to London for the purpose of having one of his pictures – a view of Nottingham – engraved.
In 1743 Sandby was appointed private secretary and draughtsman to William Augustus, Duke of Cumberland, and accompanied him in his campaigns in Flanders and Scotland (1743–1748). Sandby was at the battle of Dettingen in 1743. Pasquin says that he was appointed draughtsman to the chief engineer of Scotland, in which capacity he was at Fort William in the highlands when the Young Pretender landed, and was the first person to convey intelligence of the event to the government in 1745.
Sandby accompanied Cumberland in his expeditions against the rebels, and made a sketch of the battle of Culloden, together with three panoramic views of Fort Augustus and the surrounding scenery, showing the encampments, in 1746, and a drawing of the triumphal arch erected in St. James's Park to commemorate the victories. In this year the Duke was appointed ranger of Windsor Great Park, and selected Sandby to be deputy ranger. Sandby again accompanied the duke to the Netherlands during the War of the Austrian Succession, and probably remained there till the conclusion of the Treaty of Aix-la-Chapelle in October 1748. He drew four views of the camps in the Low Countries, covering extensive tracts of country, and another inscribed 'Abbaye près de Sarlouis'.