He began to exhibit his works on a regular basis in the early 1760s, becoming an active member of the Society of Artists,[7] which awarded him prizes for paintings of subjects from British history in 1763 and 1764.

The second of these prizes was for a picture entitled St Paul Preaching to the Ancient Druids in Britain (now in the Guildhall in High Wycombe).

[9] He was inspired by both the work and the legend of the life of the seventeenth-century Neapolitan painter, Salvator Rosa, who it was claimed, had been brought up by bandits.

[8] From 1770 to 1773 he was engaged in the decoration of the saloon at Brocket Hall, Hertfordshire, where he was assisted by Thomas Jones, Francis Wheatley, James Durno, and Burnaby Mayor.

Owing to his membership of the Society of Artists, Mortimer did not exhibit at the Royal Academy until 1778, when he showed five works, including Sir Arthegel and three scenes with Italian bandits.

Horace Walpole, the politician and writer, claimed that Mortimer was nothing more than an "imitator" of Salvator Rosa in his war scene paintings.