One of the wealthiest and most powerful men of his age, he was a central player in the conflicts between Edward II of England and his nobility, particularly Thomas, 2nd Earl of Lancaster.

His position changed with the great insult he suffered when Gaveston, as a prisoner in his custody whom he had sworn to protect, was removed and beheaded at the instigation of Lancaster.

Though earlier historians saw Pembroke as the head of a "middle party", between the extremes of Lancaster and the king, the modern consensus is that he remained essentially loyal to Edward throughout most of his career.

He is today remembered primarily through his wife Marie de St Pol's foundation of Pembroke College, Cambridge, and for his splendid tomb that can still be seen in Westminster Abbey.

[1] William was Henry III's half-brother through his mother's prior marriage to King John, and as such gained a central position in the Kingdom of England.

In 1311 the initiative known as the Ordinances was introduced, severely limiting Royal powers in financial matters and in the appointment of officers.

[18] When Gaveston without permission returned from exile later the same year, a Baronial council entrusted Pembroke and the Earl of Surrey, with the task of taking him into custody.

[19] This they did on 19 May 1312, but not long after Thomas of Lancaster, acting with the earls of Warwick, Hereford and Arundel, seized Gaveston and executed him on 19 June.

[21] As far as Pembroke was concerned, the seizing and execution of a prisoner in his custody was a breach of the most fundamental chivalric codes, and a serious affront to his honour.

[4] In 1317, however, while returning from a papal embassy to Avignon, he was captured by a Jean de Lamouilly, and held for ransom in Germany.

[24] Although ostracised because of the murder of Gaveston, Thomas of Lancaster had regained virtual control of royal government in the period after England's defeat at Bannockburn.

[30] T. F. Tout in 1914, one of the first historians to make a thorough academic study of the period, considered Pembroke the one favourable exception in an age of small-minded and incompetent leaders.

[31] Tout wrote of a "middle party", led by Pembroke, representing a moderate position between the extremes of Edward and Lancaster.

[34] Aymer married twice; his first marriage, before 1295, was to Beatrice, daughter of Raoul de Clermont, Lord of Nesle in Picardy and Constable of France.