Born at Hertford, England, on 12 October 1781, he was a student at the Royal Academy School when he was selected to be landscape painter on the ship Investigator, under FLINDERS, which sailed from Spithead on 18 July 1801. For two years he made many drawings on the ship's journey, but transferring to the Porpoise was wrecked off the coast of Queensland on a coral reef, to be rescued eight weeks later. He went on to China in the Rolla from there to Bombay, and thence to England in 1805. A few months later he sailed to Madeira and then Jamaica before returning to England, where he at once began exhibiting at the exhibitions of the Royal Academy, and from 1810 with the Old Water-Colour Society.

Flinders' A Voyage to Terra Australis (published 1814) had nine excellent large plates after Westall's drawings, and besides painting in both oil and watercolour Westall did a large number of book illustrations. His Views of Australian Scenery (1814) is, however, merely a reprint of the plates in Flinders's volume. He was elected an ARA. in 1812, but though a fairly frequent exhibitor until towards the end of his life, he never became a full academician. He met with a severe accident in 1847 which greatly affected his health, and he died at London on 22 January 1850. A large collection of his drawings is in the library of the Royal Empire Society, London.