|This striking and noble portrait of the Count was probably painted when he was made major-general in 1704; his apparent age and style of dress, particularly the wig, suggests it could not be much later than this. The confident application of paint and the subtle flesh tones together with an adroit handling of the surface armour all displays the hand of an accomplished artist. Being both realistic and animated it is of far greater quality than the mannered, stiff, portraiture produced by many parochial painters in Austria and Germany at this time and consequently suggests that this could be by an immigrant artist, possibly French. Some painters from France, such as Joseph Vivien (1657-1734), travelled extensively across the European courts for commissions and influenced many other artists in their travels. It appears this is the only known oil portrait of the Count to date, though an 18th century engraving does exist depicting him as an older man. Recently discovered, it was once in the collection at Schloss Milkel, a baroque castle in eastern Germany.
Born at Longwy in Lorraine, then a Duchy under the Holy Roman Empire, de Mercy entered the Austrian army as a volunteer in 1682 and won his commission at the Battle of Vienna the following year. During seven years of campaigning in Hungary he rose to the rank of Rittmeister and for a further five years, until 1697, he was engaged in the Italian campaigns, returning to Hungary to fight in the Battle of Zenta, after which he was quickly promoted.
De Mercy displayed great daring in the first campaigns of the War of the Spanish Succession, under Prince Eugene of Savoy, and for his conduct at the surprise of Cremona (31st January, 1702) received the thanks of Leopold I, Holy Roman Emperor, and the proprietary colonelcy of a newly raised cuirassier regiment. With this he took part in the Rhine campaign of 1703, and the Battle of Friedlingen, where his success as an intrepid leader of raids and forays became well known to both friend and foe. Selected early in 1704 to harry the dominions of Maximilian II Emanuel, Elector of Bavaria, he was soon afterwards promoted to General-Feldwachtmeister, in which rank he was engaged in the Battle of Schellenberg (2nd July , 1704). During the rest of the Succession war and the conflicts that followed he showed conspicuous gallantry and rose to be general of cavalry fighting with distinction at the Battle of Peterwardein (1716), soon afterwards being made commander of the Habsburg province Banat of Temesvar. At the Battle of Belgrade (1717) he led the second line of left wing cavalry in a brilliant and decisive charge which drove the forces of the Ottoman Empire to their trenches. After the peace he resumed the administration of the Banat, reorganizing the country as a prosperous and civilized community. However he was soon called upon again to a command in the field, this time in southern Italy, where he was engaged against Spain in the Battle of Francavilla (20th June, 1719) as part of the War of the Quadruple Alliance.
In 1734, at the age of 68, he was made a field marshal in the Austrian army and in the spring of that year, together with Frederick of Württemberg, assembled 50,000 men at Mantua in an effort to reclaim all that had been lost the previous year when the Austrians had been driven from Italy by the Franco-Spanish forces. Tragically, de Mercy was killed by musket fire on the 29th of June at the Battle of Parma while personally leading his troops.