Details of Postlethwayt's life are obscure. He was almost certainly a younger brother of the economist and writer Malachy postlethwayt and the son of Matthew Postlethwayt, vicar of Shottesham, rector of Denham and later archdeacon of Norfolk. If so, then either Shottesham or Denham is a likely Place of birth, and his date of birth can be surmised to be circa 1708–15. Like his brother, he became a Fellow of the Royal Society, probably some time in the 1740s. His works on taxation and mortality were all published in 1759, shortly before his death.
From The Biographical Dictionary of British Economists in Oxford Reference.