Scottish writer on finance and agriculture, was the eldest son. of George Sinclair of Ulbster, a member of the family of the earls of Caithness, and was born at Thurso Castle on the 10th of May 1754. After studying at Edinburgh, Glasgow and Trinity College, Oxford, he was admitted to the faculty of advocates in Scotland, and called to the English bar, but never practised. In 1780 he was returned to parliament for Caithness, and also represented several English constituencies, his parliamentary career extending, with few interruptions, until 1811.

He established at Edinburgh a society for the improvement of British wool, and was mainly instrumental in the creation of the Board of Agriculture, of which he was the first president. His reputation as a financier and economist had been established by the publication, in 1784, of his History of the Public Revenue of the British Empire; in 1793 widespread ruin was prevented by the adoption of his plan for the issue of exchequer bills; and it was on his advice that, in 1797, Pitt issued the loyalty loan of eighteen millions for the prosecution of the war. His services to scientific agriculture were no less conspicuous.

He supervised the compilation of the valuable Statistical Account of Scotland (2, vols., 1791-1799), and also that of the General Report of Scotland, issued by the Board of Agriculture; and from the reports compiled by this society he published in 1819 his Code of Agriculture. He was a member of most of the continental agricultural societies, a fellow of the Royal Societies of London and Edinburgh, as well as of the Antiquarian Society of London, and president of the Highland Society in London. Originally a thorough supporter of Pitts war policy, he later on joined the party of armed neutrality. In 1805 he was appointed by Pitt a commissioner for the construction of roads and bridges in the N. of Scotland, in 1810 he was made a member of the privy council and, next year, received the lucrative sinecure office of commissioner of excise. He died on. the 21st of December 1835.

Sir John Sinclair, who was created a baronet in 1780, was twice married, first to a daughter of Alexander Maitland, by whom he had two daughters, and secondly to Diana, daughter of the first lord Macdonald, by whom he had thirteen children. His eldest son, Sir George Sinclair (1790-1868) was a writer and a member of parliament, representing Caithness at intervals from 1811 till 1841. His son, Sir John George Tollemache Sinclair, the 3rd baronet, was member for the same constituency from 1869 to 1885. The first baronets third son, John (1797-1875), became archdeacon of Middlesex; the fifth son, William (1804-1878), was prebendary of Chichester and was the father of William Macdonald Sinclair (b. 1850), who in 1889 became archdeacon of London; the fourth daughter, Catherine (1800-1864), at one time enjoyed some vogue as an author.