From 1364 to 1372, Guichard participated in the English administration of Aquitaine and was appointed marshal. He joined the Black Prince's expedition to Spain during Castilian Civil War between 1366 and 1369 and fought in the Battle of Najera of 1367. After the Spanish campaign, he was sent to Rome. In his absence the hostilities resumed, and (according to Froissart) Guichard's embassy had to split and find the way through Savoy, Auvergne and Brittany. He joined John of Hastings, earl of Pembroke in a raid into the valley of Loire. He was a member of John Chandos' troop when Chandos was mortally wounded. Shortly afterwards, Guichard was appointed governor of Poitou. In 1370, Guichard participated in the sack of Limoges by Prince Edward's forces. He spent the winter of 1371-1372 in England, where he was made Knight of the Garter. On 22 June 1372, he was captured on the way back to the continent in the Battle of La Rochelle together with Hastings. He couldn't ransom himself out because his lands were taken by the French. He was transferred to in 1374 and was exchanged in 1375. He was appointed tutor of Richard, Prince of Wales, and at his coronation in 1377 King Richard made Guichard Earl of Huntingdon for life and granted him an additional pension.