Mortimer was a voluminous writer, chiefly of biographies and economic subjects. Born on 9th December 1730 in Carey Street, Lincoln’s Inn Fields, London, the only son of Thomas Mortimer (1706–1741), principal secretary to Sir Joseph Jekyll, master of the rolls. Educated at Harrow he precociously published in 1750 An Oration on the much Lamented Death of H. R. H. Frederick, Prince of Wales. Encouraged by its success, he taught himself French and Italian, and in 1751 he published Life and Exploits of Pyrrhus, translated from the French of J. Gautier. He married twice, the first time when he was very young, and had a large family, including two sons who served in the navy.
In 1762 he was made English vice-counsul for the Austrian Netherlands, a post he retained for six years. His publications included Every Man his Own Broker, or, A Guide to Exchange Alley (1761), based on his own experience of losing a lot of money on the stock exchange in 1756. Mortimer’s largest work was The British Plutarch, which details in six volumes the lives of eminent Britons from the time of Henry VIII to George II. He also authored a General Commercial Dictionary and translated Gautier’s Life and Exploits of Pyrrhus from the French. This three volume History seems likely to have been his proudest project, given his known preference for the subject and the ambitious title-page dedication to Queen Charlotte.