He worked as a portrait painter, an etcher, and an illustrator, but he was especially keen on history paintings for which one of his favorite sources was early British history. He also drew inspiration from writers like Shakespeare; one of his early works, for example, was A Scene from "King John" (1768). His interest in Shakespeare gave us the "Twelve Heads" in Shakespeare's Characters, imaginative pen drawings of various characters from the plays. The illustrations were first shown at a Society of Artists exhibition; Mortimer then released them as etchings in two issues, the first in 1775 and the second in 1776. Richard Altick says that after their publication in 1775-6 the heads appeared "subsequently in other forms" (256). One bibliography (compiled by James M. Hubbard for the Barton Collection of the Boston Public Library) cites a nineteenth-century edition (1885) of Shakespeare with copies of the heads bound with the plays they represented. The heads as a consequence of so many reproductions appear in a variety of sizes and shapes, some framed in a square, others in a cartouche. Both are represented in the examples listed here, but I have not indicated the sizes the sizes of all the etchings and drawings.