Mortimer was deeply in debt when he accompanied Henry V's forces to France. He took part in several campaigns in Normandy, including the Siege of Harfleur, where he contracted dysentery, and was forced to return to England. On 15 August 1416 he was appointed a captain of the expedition sent to relieve Harfleur under John, Duke of Bedford, and Sir Walter Hungerford, and was with the army which conquered Normandy in 1417 and 1418. In July 1420 he was at the siege of Melun.