Thorold was the eldest son of Sir John Thorold, 8th Baronet and his wife Elizabeth Ayton, daughter of Samuel Ayton of West Herrington, County Durham. He was born on 18 December 1734 and baptized at St James Westminster on 5 January 1735. He matriculated at
Hertford College, Oxford on 24 November 1752. His wife was Jane Hayford, daughter of Millington Hayford of Oxton Hall, Nottinghamshire and Millington Hall, Cheshire, whom he married on 18 March 1771. He succeeded his father in the
baronetcy and to
Syston Park on 5 June 1775. For 1778–9 he was
High Sheriff of Lincolnshire. ==Political career==