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 (1402), admiral of the fleet for the northern parts (1403), captain of Calais (1407), and admiral of the northern and western seas for life (1408/9). His most notable action during this decade was commanding the forces against the northern rebellion of 1405.
He was Chancellor of England (January 31, 1410 - January 5, 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 (1412).
On the accession of Henry V Beaufort was appointed Lieutenant of Aquitaine (1413) and then captain of Harfleur (1415). He spent the next years in Normandy, being Lieutenant of Normandy (1416). He was created Duke of Exeter for life (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 Evreux, 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 Chateau-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, and 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 Bagué where his nephew Thomas of Lancaster was killed.
Beaufort was one 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's likely he spent some time in France as well.
He married a Margaret Neville (there were a lot of Margarets and a lot of Nevilles around the royal circle then, but this one was the daughter of Sir Thomas Neville of Horneby). They had only one child, Henry Beaufort, and he died young.