Mortimer was born in Eastbourne, where his father owned a mill, but came as a young man to London, where he studied under Thomas Hudson and Robert Edge Pine as well as at the Duke of Richmond’s Gallery and the St Martin’s Lane Academy. He won five premiums from the Society of Arts between 1759 and 1762 and was estimated to hold great promise. He was elected President of the Society of Artists in 1774. Redgrave states that he led a rather dissolute life with bohemian friends in the purlieus of Covent Garden, but reformed after his marriage when he left town to live in Aylesbury. As well as conversation pieces and portraits, Mortimer specialized in history painting and romantic subjects portraying banditti in the manner of Salvator Rosa. He perhaps never reached his full potential, dying at the age of 39 after a short fever.