In August 1436 Mowbray accompanied the Duke of Gloucester on a campaign to relieve Calais, then under siege by Philip the Good, Duke of Burgundy. The expedition, in which Mowbray provided a contingent, was "one of the largest English armies assembled during the fifteenth century." The campaign was a success, and Burgundy was forced to withdraw. On 13 September that year, Mowbray received livery of his inheritance, and immediately began a busy period devoted to royal service. In 1437, possibly because of Gloucester's patronage, Mowbray was appointed Warden of the Eastern March for a one-year term. He had little experience of the north of England, yet was paid wartime wages of £5,000 to campaign against the Scots.