Almost continually engaged in the wars in France, he was seriously wounded during the Siege of Harfleur (1415), where his father died from dysentery. Later that year his older brother Michael, 3rd Earl of Suffolk, was killed at the Battle of Agincourt, and William succeeded as 4th Earl. He served in all the later French campaigns of the reign of Henry V, and in spite of his youth held high command on the marches of Normandy in 1421–22. In 1423 he joined Thomas, Earl of Salisbury in Champagne. He fought under John, Duke of Bedford, at the Battle of Verneuil on 17 August 1424, and throughout the next four years was Salisbury's chief lieutenant in the direction of the war. He became co-commander of the English forces at the Siege of Orléans (1429), after the death of Salisbury. When that city was relieved by Joan of Arc in 1429, he managed a retreat to Jargeau where he was forced to surrender on 12 June. He was captured by a French squire named . Admiring the young soldier's bravery, the earl decided to knight him before surrendering. This dubbing has remained famous in French history and literature and has been recounted by the writer Alexandre Dumas. He remained a prisoner of Charles VII of France for three years, and was ransomed in 1431, after fourteen years' continuous field service.