Mortimer was born in London on 9 December 1730, and died there on 31 March 1810. His father was a lawyer and secretary to the Master of the Rolls. He was educated at Harrow and then, following his father's death in 1741, at a private school in the north of England. Having inherited an ample income, he devoted himself to study and scholarship, especially of languages and modern history. He also dabbled in the stock exchange, losing by his own account a great deal of money in 1756 when an investment went sour, and thereafter he was faced with the need to earn a living. In 1762 at the recommendation of the Earl of Sandwich, then Secretary of State, Mortimer was appointed British vice-consul in the Austrian Netherlands, and took up residence at Ostend. He held this post until 1768, when he was suddenly dismissed following a political intrigue; the exact details of this are unclear, but Mortimer's supporters spread a rumour that he had been too ardent in his anti-Jacobite sentiments.
From The Biographical Dictionary of British Economists in Oxford Reference.