Politician, lawyer and agricultural reformer. Born in Thurso Castle and educated at Edinburgh, Glasgow and Oxford, Sinclair worked as a Barrister both in Scotland and England (1775-82). He was elected Member of Parliament for Caithness in 1780; for Lostwithiel in 1784; and for Petersfield from 1796 to 1811.
He was the first president of the Board of Agriculture in the administration of William Pitt, the Younger, effectively founding the government's Department of Agriculture. His chief works were his History of the Revenue of the British Empire (1784) and his Statistical Account of Scotland, published in twenty-one volumes between 1791 and 1799. The fore-runner of the modern census, this was the first systematic attempt to compile social and economic statistics for every parish in the country. He took his information from individual parish ministers, with the support of the General Assembly of the Church of Scotland, of which Sinclair was a lay member. Today, the account provides a unique record of life at the end of the 18th C.
From 1815, Sinclair lived on George Street (Edinburgh) where he died. He is buried in the ruins of Holyrood Abbey. His statue is prominently displayed in Thurso in appreciation of the work he undertook to improve the town.
