Ewart was born on 30 April 1759, the eldest son of Rev. John Ewart, the minister of Troqueer in Kirkcudbrightshire, and his wife Mary Corrie. He was educated at Dumfries Parish School then sent to Edinburgh University, and then acted as travelling tutor to Macdonald of Clan Ronald.
While abroad, Ewart made the acquaintance of Sir John Stepney, British minister at Dresden, and after that diplomat was transferred to Berlin, Ewart became his private secretary and then secretary of legation. After acting as chargé d'affaires from 1787 to 1788, he was appointed envoy extraordinary and minister plenipotentiary to the King of Prussia on 5 August 1788.
Service in Prussia
Pitt's design, assisted by Lord Malmesbury, was to induce Frederick William of Prussia to intervene in the affairs of Holland; to put down the revolutionary party there; and to re-establish the Prince of Orange as a stadt-holder. This design was carried out, and Ewart obtained much credit for his share in the transactions. Of his subsequent conduct at the court of Berlin there are contradictory reports, for the French revolution commenced in 1789, and partisans and opponents of the English foreign policy of that period represent the minister's behaviour in different lights. Ewart was accused of adopting too peremptory an attitude towards the King of Prussia and his ministers, thus alienating them from England. He succeeded in concluding the marriage treaty between the Duke of York and the eldest daughter of the King of Prussia, and received warm acknowledgments from the king. His health breaking down, he resigned on a pension of 1,000l. a year and a promise of the Order of the Bath. He left Berlin on 3 November 1791. He died at his brother's house in Bladud's Buildings, Bath, on 27 January 1792, and was buried in Bath Abbey, where a tablet is eructed to his memory. A statement that he died out of his mind, and another (by Wraxell) that his death was due to foul play of the Empress Catherine, are entirely disproved by facts preserved in the family papers.
In 1785 he married the Countess Wartensleben, by whom he left one son (afterwards Lieutenant-general Ewart, C.B.) and two daughters.
- BIOGRAPHICAL INDEX OF FORMER FELLOWS OF THE ROYAL SOCIETY OF EDINBURGH 1783 – 2002 (PDF). The Royal Society of Edinburgh. July 2006. ISBN 0 902 198 84 X.