He was the founder of the cadet dynasty known as "Courtenay of Powderham", seated at the manor of Powderham, until then a former Bohun manor of little importance, whilst the line descended from his elder brother, the Earls of Devon of the mediaeval era, continued to be seated at Tiverton Castle and Okehampton.

Sir Philip Courtenay was the fifth son of eight sons of Hugh Courtenay, 10th Earl of Devon (1303–1377) and Margaret de Bohun, daughter of Humphrey de Bohun, 4th Earl of Hereford by his wife Elizabeth Plantagenet (1282–1316), a daughter of King Edward I.

He was knighted before the battle with his brothers Peter Courtenay, KG, and Hugh Courtenay, KG[3] On his return, Sir Philip was made Admiral of the West on 7 October 1372 responsible for defending a coast plagued by pirates he held that post until 16 July 1376.

Back in England, he launched a petition to parliament protesting against the corruption of the Stannaries, which he claimed was preventing the protection and patrol of the coastline.

That August, Philip was engaged with Peter in a desperate battle against the Spaniards during which they were both captured as supplies failed to get through to Gascony.

John of Gaunt opposed any such venture, whereas the clerical party was supported by Henry le Despencer, Bishop of Norwich and Sir Philip Courtenay.

There in Hampshire they attacked and tortured the Carmelites, and when they accused Gaunt of treason, the knights caused the death of the friar.

Richard II demanded his presence in parliament because he had been given the official post of Keeper of Dartmoor forest in 1370, and granted substantial estates in Devon from 1380.

In 1389 he was given the post of Lord Warden of the Stannaries and granted the Royal Manor of Haslebury Plucknett for a period of six years.

In December 1404, the king ordered Philip to grant Dartmoor and Bradninch to Henry, Prince of Wales.

They urged Henry IV to bring the experience of the West Country men in provisioning stores against France in 1402.

[4] Many men complained of Courtenay's conduct: Sir Thomas Pomeroy was returned some property lost in Exeter in 1402.

In 1377, Sir Philip returned advowsons of Honiton and East Coker to Exeter Cathedral to construct a memorial to his father.

[6] He was succeeded by his son, Richard, Lord Chancellor and Bishop of Ireland, upon his death on 29 July 1406.