In 1183 he appears as a leader of Brabant mercenaries in Southern France. He entered King Richard's service in 1184, attacking and laying waste to lands of Aimar V of Limoges. In 1188 he managed seventeen castles captured from the Count of Toulouse. He accompanied Richard on the Third Crusade but was sent back when Philip Augustus left for France. On arrival he and his mercenaries defended Richard's estates in the latter's absence.
After Richard's return from the Holy Land, Mercadier accompanied him everywhere as his right hand, travelling and fighting by his side. Richard eulogized Mercadier's exploits in his letters, and gave him the estates left by Ademar de Bainac in Limousin, who died without heirs around 1190. During the various wars between Richard and Philip Augustus of France, Mercadier fought successively in Berry, Normandy, Flanders and Brittany. When Richard was mortally wounded at the siege of Châlus in March 1199, it was Mercadier's physician who cared for him. Mercadier avenged his death by storming the castle, hanging the defenders and flaying Bertrand de Gourdon, the crossbowman who had shot the king, despite Richard's last act pardoning him.
Mercadier then entered the service of Eleanor of Aquitaine, and ravaged Gascony and the city of Angers. On Easter Monday, 10 April 1200, he was assassinated while on a visit to Bordeaux to pay his respects to Eleanor of Aquitaine, who was bringing from Spain Blanche of Castile. His murder was at the hands of six men-at-arms employed by Brandin, a rival mercenary captain in the service of Richard's successor King John Lackland.
One of the bridges of the Château Gaillard (built by his employer King Richard) is named for him.
In historical fiction
Books and dramas
Five novels "Les aventures de Guilhem d’Ussel" of Jean d'Aillon During the reign of Philip August, Guilhem d’Ussel encounters several chiefs of mercenaries : Mercadier, Lambert Cadoc (Lord of Gaillon) and Brandin.
- De Taille et d'Estoc (The youth of Guilhem d'Ussel)
- Marseille, 1198
- Paris, 1199
- Londres, 1200
- Montségur, 1201
Mercadier also made an appearance in Robin and Marian, the 1976 film serving as right hand to Richard the Lionheart. Played by Bill Maynard.
- This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Mercadier". Encyclopædia Britannica (11th ed.). Cambridge University Press.
- "Mercadier", in Bibliothèque de l'École des Chartes, 1st series, t. iii., pp. 417–443.
- The Art of Warfare in Western Europe during the Middle Ages from the Eighth Century (Warfare in History) by J. F. Verbruggen, pp. 116–117*