William Essex trained as an enamel painter in the workshop of Charles Muss (1779 – 1824), Enamel Painter to William IV. Essex first exhibited at the Royal Academy in 1818 and throughout the 1820s and early 1830s built up a successful practice, working chiefly in enamel. William Essex must have had his first introduction to court circles through Charles Muss. He may initially have worked in collaboration with his teacher, but by 1827 was earning his own commissions from George IV. In 1834 Essex painted an enamel miniature of Charlotte, Duchess of Northumberland, governess to Queen Victoria, after Sir Thomas Lawrence; although he was already well established in royal circles, his patronage by the Duchess of Northumberland may have been an additional factor that contributed to his appointment as Enamel Painter to Queen Victoria in 1837 and Enamel Painter to Prince Albert in 1841. Queen Victoria employed Essex to make numerous enamel copies of portraits of her after Franz Xaver Winterhalter, which were set into bracelets and distributed as gifts. He also made numerous copies of portraits of her relatives and contemporaries. Most of these were commissioned within the same year as the original on which they were based, and were often given by Queen Victoria to Prince Albert. He also made historical copies of early miniatures and portraits at Windsor. Despite Essex's ostensible success, however, he fell into poverty in his later years and was forced to request that the Queen supplement his artist's annuity of £40 with a pension.

Essex's enamel is after a portrait type by Franz Krüger (1797 – 1857) showing the King of Hanover full-length, standing in uniform, in an interior. A signed version dating from 1840 was in the Hanover sale, Schloss Marienburg, Sotheby's, 5 – 15 October 2005 (lot 1704). When the Duke of Cumberland attended the christening of Princess Alice in 1843, Queen Victoria found him 'very much changed, so shrunk & bent, so aged & all his hair gone' (Journal, 2 June 1843).

Ernest Augustus, Duke of Cumberland, King of Hanover, fifth son of George III and Queen Charlotte, spent his childhood at Kew before following a military career with the Prussian army. He married, in 1815, his cousin Frederica of Mecklenburg-Strelitz. On the death of his brother, William IV, he inherited the Hanoverian throne under Salic law, and was succeeded by his only son, the blind George V, King of Hanover.

Signed, dated and inscribed on the counter-enamel in black paint: Ernest Aug s. / King of Hanover. / Painted by W. Essex / AD 1846 Enamel painter / to Her Majesty & H.R.H. / Prince Albert