Neville's first military service was in
Brittany under
King Richard II's uncle,
Thomas of Woodstock, who knighted him at
Saint-Omer in July 1380. On 14 November 1381 he and his cousin,
Henry "Hotspur" Percy, were commissioned to preside over a duel between an Englishman and a Scot, and on 1 December 1383 he and his father were commissioned to receive from the Scots 24,000 marks for the ransom of
King David. On 26 October 1385 he was appointed joint
Governor of Carlisle with
Sir Thomas Clifford, and on 27 March 1386 was appointed, together with Clifford, joint
Warden of the
West March. Neville inherited the title at the age of 24 after his father's death on 17 October 1388, and was summoned to Parliament from 6 December 1389 to 30 November 1396 by writs directed to
Radulpho de Nevyll de Raby. On 25 October 1388, he was appointed, with others, to survey the fortifications on the Scottish border, and on 24 May 1389 was made keeper for life of the royal forests north of the
Trent. In 1393 and 1394 he was employed in peace negotiations with Scotland. In 1397 Neville supported King Richard's proceedings against Thomas of Woodstock and the
Lords Appellant, and by way of reward was created
Earl of Westmorland on 29 September of that year. However his loyalty to the King was tested shortly thereafter. His first wife, Margaret Stafford, had died on 9 June 1396, and Neville's second marriage to Joan Beaufort before 29 November 1396 made him the son-in-law of King Richard's uncle,
John of Gaunt, 1st Duke of Lancaster. Thus, when King Richard banished John of Gaunt's eldest son and heir, Henry Bolingbroke, on 16 September 1398, and confiscated Bolingbroke's estates after John of Gaunt's death on 3 February 1399, Westmorland was moved to support his brother-in-law. Bolingbroke landed with a small force at
Ravenspur in July 1399. Westmorland and the
Earl of Northumberland were in the deputation at the
Tower which received King Richard's abdication, and Westmorland bore the small sceptre called the "virge" at Bolingbroke's coronation as
King Henry IV on 13 October 1399. For his support of the new King, Westmorland was rewarded with a lifetime appointment as
Earl Marshal on 30 September 1399 (although he resigned the office in 1412), a lifetime grant of the
honour of Richmond on 20 October (although the grant was not accompanied by a grant of the title
Earl of Richmond), and several wardships. Before 4 December he was appointed to the King's council. In March 1401, Westmorland was one of the commissioners who conducted negotiations for a marriage between the King's eldest daughter,
Blanche of England, and
Louis, son of
Rupert, King of the Romans, and in 1403 was made a
Knight of the Garter, taking the place left vacant by the death of
Edmund of Langley, 1st Duke of York. and led Scrope and his allies to believe that their demands would be accepted and their personal safety guaranteed. Once Scrope's army had been disbanded on 29 May, Scrope, Mowbray and Plumpton were arrested, summarily condemned to death for treason, and beheaded outside the walls of York on 8 June 1405. Although Westmorland handed Scrope and his allies over to the King at
Pontefract, he played no role in their hasty and irregular trial and execution, having been sent north by the King on 4 June to seize Northumberland's castles. It is unclear whether Northumberland had initially planned to rebel openly in concert with Scrope, but in the event, he gave Scrope no support and fled to Scotland after his failed attempt to capture Westmorland. His estates were subsequently forfeited to the crown, and Ralph, earl of Westmorland, as a reward for his quelling of the 1405 rebellion without significant bloodshed, received a large grant of former Percy lands in
Cumberland and
Northumberland in June 1405. After the death of Henry IV Westmorland was mainly engaged in the defence of the northern border in his capacity as
Warden of the West March (1403–1414). In 1415 he decisively defeated an invading Scottish army at the
Battle of Yeavering. Westmorland played no part in
King Henry V's French campaigns, and Tuck notes that his relationship with Henry V was not close, perhaps partly because of the involvement of Westmorland's son-in-law,
Sir Thomas Grey of Heaton, in the
Southampton Plot. After Henry V's death, Westmorland was a member of the
Council of Regency during the minority of
King Henry VI. According to Tait, Westmorland was "no inconsiderable builder", citing his rebuilding of
Sheriff Hutton Castle on a scale so magnificent that
Leland saw "no house in the north so like a princely lodging", his doubling of the entrance gateway of
Raby Castle and the corresponding tower, and possibly his responsibility for the "tall and striking tower" of Richmond parish church. On 1 November 1410 Westmorland was granted licence to found a college for a master, six clerks, six "decayed gentlemen" and others at
Staindrop, towards the completion of which he left a bequest in his will. ==Marriages and issue==