Lawrence was the most fashionable and also the greatest portraitist of his generation. He was made Principal Painter to George III in 1792 after Reynolds’s death, and received occasional commissions; however it was only after 1814 that George IV began to employ him in earnest.

This portrait was commissioned by George IV at a cost of 500 guineas and was painted in Rome in 1819, though it remained in Lawrence's studio until his death. The portrait seems to have always been intended for what became the 'Waterloo Chamber' and celebrates the sitter’s role as the Pope’s representative at the Congress of Vienna, where he persuaded the allies to restore the Papal States in their entirety (with the exception of Avignon). The sitter appears in Cardinal's robes with his biretta and a copy of his 1816 constitution from the Papal States in his right hand; in the background a view of St Peter's, Rome.

The Waterloo Chamber is a great hall on the public route at Windsor Castle displaying portraits of those soldiers, sovereigns and diplomats responsible for the overthrow of Napoleon and the re-establishment of the monarchies and states of Europe thereafter. The concept began in 1814 when George IV used the opportunity of the Treaty of London to commission Lawrence to paint distinguished visitors. The group of portraits grew during the next decade as Lawrence continued to obtain portrait sittings at the various congresses following the Battle of Waterloo in 1815 and, in some cases, by making special journeys. Most of the twenty eight portraits were delivered after his death on 7 January 1830. By this time work was already begun of the space of the Waterloo Chamber created by covering a courtyard at Windsor Castle with a huge sky-lit vault; the room was completed during the reign of William IV (1830-7). The first illustration of the interior is provided by Joseph Nash (1809-78) in 1844 (RCIN 919785) and shows the arrangement which survives to this day: full-length portraits of warriors hang high, over the two end balconies and around the walls; at ground level full-length portraits of monarchs alternate with half-lengths of diplomats and statesmen. This portrait is out of step in being the only full-length of a statesman.