After the accession of his half-brother Henry IV, Beaufort was made a Knight of the Garter. In the following years he held various military posts: constable of Ludlow, appointed Admiral of the North, appointed captain of Calais, and Admiral of the North and West he retained the title for life. His most notable action during this decade was commanding the forces against the northern rebellion of 1405. He became Chancellor of England on 31 January 1410, an office he held until 5 January 1412 during a time when King Henry was having trouble with the clergy, and then returned to military matters. Later in 1412 he was created Earl of Dorset.
On the accession of Henry V, Beaufort was appointed Lieutenant of Aquitaine and then captain of Harfleur. He spent the next years in Normandy as Lieutenant of Normandy. He was created Duke of Exeter for life in 1416. Beaufort was back in England in 1417, while the king was in Normandy, but had to deal with problems in Scotland. In 1418 he went back to Normandy with a large force, taking part in the sieges of Évreux, Ivry, and Rouen. After the fall of Rouen in 1419, he was captain of the city and conquered more of the smaller Norman cities. Finally, in 1419, he took the great fortress of Château Gaillard, midway between Rouen and Paris, after a six-month siege. During this time, Henry V had a policy of creating Norman titles for his aristocrats, thus Beaufort was created Count of Harcourt in 1418. In 1420, Beaufort helped negotiate the treaty of Troyes. The next year he was captured at the Battle of Baugé, where his nephew Thomas of Lancaster, 1st Duke of Clarence, was killed. Beaufort was one of the executors of Henry V's will, and so returned to England in 1422. He served on the governing council for the infant king Henry VI, though it is likely he spent some time in France as well. The character of Exeter in Shakespeare's play Henry V is based on Beaufort, although Beaufort was not actually created Duke of Exeter until after the Battle of Agincourt. In 1415 he was Earl of Dorset. He died on 31 December 1426. 27 December 1426 or 1 January 1427 have been suggested as alternative dates.
Arms
As a legitimated grandson of the sovereign, Beaufort bore the arms of the kingdom, differenced by a bordure gobony azure and ermine.