The 4th Baron died on 9 August 1396, and Willoughby inherited the title as 5th Baron. He was given
seisin of his lands on 27 September. Hicks notes that the Willoughby family had a tradition of military service, but that the 5th Baron 'lived during an intermission in foreign war and served principally against the Welsh and northern rebels of
Henry IV'. Willoughby joined Bolingbroke, the future King Henry IV, soon after his landing at
Ravenspur, was present at the
abdication of
Richard II in the
Tower on 29 September 1399, and was one of the peers who consented to King Richard's imprisonment. In the following year he is said to have taken part in Henry IV's expedition to Scotland. In 1401 he was admitted to the
Order of the Garter, and on 13 October 1402 was among those appointed to negotiate with the Welsh rebel,
Owain Glyndŵr. When Henry IV's former allies the Percy Family rebelled in 1403, Willoughby remained loyal to the King. In the July of that year, he was granted lands that had been in the custody of
Henry Percy, who had been killed at the
Battle of Shrewsbury on 21 July 1403. Willoughby was appointed to the King's council in March 1404. On 21 February 1404 he was among the commissioners appointed to expel aliens from England. In 1405 Hotspur's father,
Henry Percy, 1st Earl of Northumberland, again took up arms against the King, joined by
Lord Bardolf, and on 27 May
Richard Scrope,
Archbishop of York, perhaps in conjunction with
Northumberland's rebellion, assembled a force of some 8,000 men on
Shipton Moor. Scrope was tricked into disbanding his army on 29 May, and he and his allies were arrested. Henry IV denied them trial by their peers, and Willoughby was among the commissioners who sat in judgment on Scrope in his own hall at his manor of Bishopthorpe, some three miles south of
York. The Chief Justice, Sir
William Gascoigne, refused to participate in such irregular proceedings and to pronounce judgment on a prelate, and it was thus left to the lawyer Sir William Fulthorpe to condemn Scrope to death for treason. Scrope was beheaded under the walls of York before a great crowd on 8 June 1405, 'the first English prelate to suffer judicial execution'. On 12 July 1405 Willoughby was granted lands forfeited by the rebel Earl of Northumberland. In 1406 Willoughby was again appointed to the Council. On 7 June and 22 December of that year he was among the lords who sealed the settlement of the crown. ==Marriages and issue==