He served in France under the earl of Arundel in March 1344 and in August 1346 was at the Battle of Crécy. He then transferred to service in Gascony under the earl of Lancaster. In July 1352 he was made joint warden of the marches towards Scotland, and in September 1355 keeper of Roxburgh Castle and sheriff of Northumberland for two years.
In 1356 he took part in the invasion of Scotland that followed Edward Balliol's surrender of the kingdom and crown of Scotland to Edward III. In July 1356 he was once again joint warden of the marches and then took part in the negotiations that led to the treaty of Berwick of October 1357.
He participated in Edward III's military efforts in France and in September 1355 was marshal of the royal army at Calais. In 1360 he took part in Edward's campaign to capture Rheims.
He died on or about 18 May 1368, probably at his castle of Alnwick, and was buried in Alnwick Abbey. He had married twice: firstly Mary of Lancaster (d. 1362), daughter of Henry, 3rd Earl of Lancaster, and secondly Joan (d. 1368), the daughter and sole heiress of John, Lord Orreby. With his first wife he had two sons, Henry and Thomas. With his second wife he had a son who died in his father's lifetime and a daughter, Mary. He was succeeded by his eldest son Henry, who became the 1st earl of Northumberland. His younger son Thomas became the 1st Earl of Worcester.
- Bean, J.M.W. "Percy, Henry, third Lord Percy (c.1321–1368)". Oxford Dictionary of National Biography (online ed.). Oxford University Press. doi:10.1093/ref:odnb/21930. (subscription or UK public library membership required)
|NAME||Percy, Henry, 3rd Baron of Alnwick|
|SHORT DESCRIPTION||English baron and High Sheriff of Northumberland|
|DATE OF BIRTH||c. 1321|
|PLACE OF BIRTH|
|DATE OF DEATH||1368|
|PLACE OF DEATH|
|Offspring of Henry, 3rd Baron Percy of Alnwick and Mary of Lancaster (c1320-1362)|
|Henry Percy, 1st Earl of Northumberland (1341-1408)||10 November 1341 England, United Kingdom||20 February 1408 Bramham Moor, Yorkshire, England|| Margaret Neville (1329-1372)|
|Thomas Percy, 1st Earl of Worcester (1343-1403)||1343||23 July 1403 Shrewsbury, Shropshire, England, United Kingdom|
|Offspring of Henry, 3rd Baron Percy of Alnwick and Joan Orreby (-1368)|
|Mary Percy (c1365-)|
|Offspring of Henry, 2nd Baron Percy of Alnwick and Idoine de Clifford (c1300-1365)|
|Henry de Percy, 3rd Baron Percy (1320-1368)||1320||May 1368 England, United Kingdom|| Mary of Lancaster (c1320-1362)|
Joan Orreby (-1368)
|Thomas Percy, Bishop of Norwich|| |
|Roger Percy (c1324-)|| |
|Maud Percy (-1379)||18 February 1379|| John Neville, 3rd Baron Neville de Raby (1328-1388)|
|Eleanor Percy (c1328-)|| |
|Isabel Percy (c1330-)|