Robert de Vere, 5th Earl of Oxford and 6th Great Chamberlain, was a follower of Simon de Montfort, Earl of Leicester, who knighted him on the field of battle in 1264, and summoned him to the parliament of 1265. Just a few days before the battle of Evesham Robert and young Hugh de Montfort were attacked at Kenilworth and taken prisoner. The Battle of Evesham, August 4, 1265, erupted when an alliance of rebellious barons under Simon de Montfort attacked royal forces under Prince Edward, who would later be crowned Edward I. Montfort and the rebels were incensed by poor governance and outrageous spending on foreign wars by Henry III. Montfort's success at the Battle of Lewes had given him de facto control of the government, and he pushed through reforms that led eventually to the parliamentary system of government by representation. The plan, however, came undone by his dissention among the rebel allies, and Prince Edward escaped from captivity to raise the royal flag in Wales. Simon de Montfort intended to join forces with his son Hugh at Kenilworth Castle, near Warwick. But Prince Edward arrived first, arresting Hugh Montfort and Robert de Vere, and surprising Simon on his arrival at Evesham. Legend has it that Montfort, seeing the trap his army was in, said, "Let us commend our souls to God, because our bodies are theirs." Montfort's precarious situation was made worse when his Welsh allies deserted just before battle began, leaving him facing an enemy four times the size of his own. Montfort's men bravely charged against the royal lines, but the Princes troops converged on Montfort's flanks and there was massacre. Even the deserting Welshmen were chased down and slaughtered. Montfort's body was torn apart, and parts were sent to different parts of the kingdom. His torso was stashed at Evesham Abbey, where his tomb later became a popular destination for pilgrims.

Henry III was restored to power and the remaining sons of Montfort fled the country. Earl Robert made his peace with the Crown, under the "Dictum de Kenilworth," and shortly thereafter was employed by King Edward I (Longshanks) in further battles against the Welsh.

Robert's marriage to Alice Saundford (also spelled Saunford or Sanford), the heiress of Gilbert de Sanford, brought to the Vere family the office of Chamberlain to the Queen (a role that Gilbert had exercised in 1236, when the earl's father had similarly acted as chamberlain to the king). Through this marriage, the later Earls of Oxford were able to include, in their list of titles, that of "Lord Sanford."

Robert and Alice had, with other issue, the following children:
1. Robert de Vere, his successor,
2. Alphonsus de Vere, married Jane, daughter of Richard Foliot, Knight, and had a son, John de Vere, who succeeded as 7th Earl of Oxford and 8th Great Chamberlain,
3. Hugh de Vere, Baron Vere,
4. Joane Vere, (who married William de Warren), and
5. Lora, married Reginald de Argentein.

During the 1290s King Edward I gave the De Veres permission to hold a fair and there has been a yearly carnival in the Vere area of the shire of Essex ever since.

Earl Robert died in 1296, and was succeeded by his son, Robert.