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.

William Essex's enamel is a copy after William Ross's watercolour sketch of the Count dated 1842 (913790). On the occasion of the Mensdorff-Pouillys' visit to London in May 1842, Queen Victoria wrote to Leopold I, King of the Belgians: 'Uncle Mensdorff is quite delightful & it is truly touching to see the father's love for the 4 sons & the sons love for him & for one another. Albert loves them as brothers having long known them & so do I & I hope it may be in my power to assist them. Alexander is the best of them all – he is so excellent & solid. Alphonse tho' very fat is very clever & amusing & Arthur very dear & funny - & Hugo very kind & good'.

Count Alphonse Mensdorff-Pouilly was the second son of Count Emanuel Mensdorff-Pouilly, and of Sophie, daughter of Francis, Duke of Saxe-Coburg-Saalfeld, and first cousin of Queen Victoria and Prince Albert. He married, on 22 July 1843, Countess Theresa Rosa Francesca Dietrichstein-Proskau. It was, according to Queen Victoria, 'the happiest, most loving "ménage" possible' (Journal: 30 December 1856), but the countess died from scarlet fever in 1856, leaving Count Alphonse 'sadly altered & aged' (Journal: 22 July 1858). He outlived her by almost 40 years, dying aged eighty-five on 11 December 1894.

Signed and inscribed on the counter-enamel in red paint: Count Alphonse Mensdorff / B: 24th June 1810 / Painted by W. Essex after / a sketch by Sir W. Ross / 1842 and signed and dated in yellow paint: Painted by W. Essex / 1842.