Little is documented about Harriott's life or family, but it appears that his mother was the talented 18th-century miniaturist Diana Hill (c.1760–1844), whose second marriage was to Major Thomas Harriott. Hill travelled to India with her husband, where William was born around 1790, and where Diana became one of the most prominent miniaturists in India. The family returned to England in 1806 and took up residence at West Hall in Kew. William Henry Harriott died relatively young but Diana Hill lived into old age.

William Henry Harriott was a good friend of leading watercolourist John Sell Cotman (1782–1842). Harriott, a clerk in the War Office, would spend several months each year on the Continent where he would industriously produce a large number of sketches and drawings. Harriott's Continental drawings were often elaborately detailed, owing much to the topographical work of Samuel Prout—from whom Harriott received instruction at Dr William Glennie’s academy in Dulwich Grove.

Cotman admired these so much that Harriott allowed him to make tracings of them for his own use.