After the Lancastrian usurpation of the English throne in 1399, his career in their service was transformed as he rose to national prominence, and through his access to royal patronage he acquired great wealth and influence.

Bolingbroke rewarded Erpingham by appointing him as constable of Dover Castle and warden of the Cinque Ports, and after ascending the throne as Henry IV he made him chamberlain of the royal household.

In 1415 Erpingham was indentured to serve as a knight banneret, and joined Henry's campaign to recover his lost ancestral lands in France and Normandy.

In the summer of 1380 he was indentured into the retinue of John of Gaunt, Duke of Lancaster, a military leader and the third surviving son of Edward III of England, with whom Salisbury had recently served.

John I of Portugal joined with Lancaster in March 1387, but because of a lack of food for their animals, and the successful defensive tactics employed by the Castilians, their campaign was abandoned after six weeks.

[22] In 1388, Erpingham participated before Charles VI of France in a jousting tournament at Montereau, his adversary being Sir John de Barres.

[23] As related by the French chronicler Jean Froissart, half way through the tournament, Erpingham was struck violently on his shield by his opponent, and was knocked off his horse.

[25][26] In 1390 he was with Bolingbroke's retinue when it crossed the English Channel with the intention of joining Duke Louis II of Bourbon in a siege of the Tunisian port of Mahdia on a crusading expedition via Marseilles.

[30] The historian Helen Castor has described the Lancastrian presence in East Anglia as a "disparate collection” that lacked coherence or a single identity.

Those assembled were told that the trial had been stopped to avoid dishonouring the loser and to prevent a feud from arising, but chroniclers (writing after Henry IV's accession) considered Richard's decision an act of revenge.

[42] On 17 June 1399, Erpingham witnessed a secret pact made in Paris between Bolingbroke and Louis I, Duke of Orléans, the brother of Charles VI, stating that as allies they would support each other against each other's enemies—the kings of England and France excepted.

[44] Whilst Bolingbroke was gaining support for his cause to restore his rightful inheritance of the Duchy of Lancaster as he moved across northern and central England, Richard was delayed in Ireland.

Shortly afterwards, Erpingham arrested Henry le Despenser, bishop of Norwich and one of the few remaining supporters of Richard prepared to resist Bolingbroke.

[47] The King was persuaded by the Earl of Northumberland to leave Conwy and travel 17 miles (27 km) to Rhuddlan Castle,[49] but during the journey his party was ambushed and he was taken prisoner.

[38] When Richard saw armed men everywhere, Northumberland's plans were revealed to him, and: "As he spoke, Erpingham came up with all the people of the Earl, his trumpets sounding aloud.

[50][note 5] He was a commander in the army that suppressed the Epiphany Rising of 1399–1400, led by the duketti (the disparaging term given to a large group of noblemen, many of whom had received titles from Richard).

[56] Despite the military nature of the office of constable of Dover, Erpingham took little part in the warfare of the early years of Henry IV's reign, and he generally remained at court.

[57] He campaigned in Scotland in August 1400, when Henry made a futile attempt to make the Scots acknowledge him as king of England and pay him homage.

[63] Gentry from East Anglia who were associated with Erpingham benefited from his powerful position at court: Sir John Strange of Hunstanton became controller of the royal household in 1408; Sir Robert Gurney of Gunton became Erpingham's deputy at Dover Castle in 1400; and John Winter of Barningham became controller of Prince Henry's household in 1403.

[64] The principal citizens of Norwich had become disillusioned with Richard II's policies, the city having lost its charter in 1388 when it supported the Lords Appellant.

[73] Strategic planning for the expedition in February 1415 involved discussions with Erpingham and other soldiers in Henry's inner circle, part of what the historian Anne Curry describes as the King's "strong infrastructure and amply supply of manpower".

The English successfully forded the River Somme at Voyennes; two days' march short of Calais, they were blocked by the French near Agincourt.

And thus the knight, who rode with two others only in front of the battalion, seeing that the hour was come, for all things were well arranged, threw up a baton which he held in his hand, saying " Nestrocq," which was the signal for attack; then dismounted and joined the king, who was also on foot in the midst of his men, with his banner before him.After the French army failed to attack, Erpingham was ordered to warn the army that it was about to advance to within bowshot of the French.

This plan failed when the cavalry were halted by the storm of arrow fire and the stakes planted by the archers; their retreat was disrupted by the advancing French foot soldiers.

[99] On his return to England, Erpingham's reward for the services he rendered had during the war included the farm of Lessingham manor and an annuity from the King of 50 marks.

[100][note 10] In July 1416, in his capacity as the steward of the royal household, he travelled back to Calais with John Wakering, the bishop of Norwich.

[104] Erpingham's connections with the Lancastrians and his increasing wealth led to his acquisition of lands, rents and services in Norfolk, Suffolk and Essex, manors sometimes being held in joint possession with his neighbours or relatives.

[51] Erpingham had a profound influence on the careers of the two sons of his sister Julian, who married Sir William Phelip (or Philip) of Dennington.

Kenneth Branagh, in his 1989 film, used the character more often and in, according to Curry, in a way that was "notably more inventive" than Olivier and showed more of an awareness of Erpingham's place in history.

[127] Identifiable in both films by his distinctive coat of arms and his white hair—in contrast with that of the youthful looking Henry and his courtiers—Branagh includes Erpingham to good effect in the court scenes set in England, as well as during the battle and its aftermath.