He played an important part in Edward's administration as diplomat and military commander and in 1321–22 helped suppress a rebellion.

In 1326, Edmund joined a rebellion led by Queen Isabella and Roger Mortimer, whereby King Edward II was deposed.

Once the new king, Edward III, came of age and assumed personal control of the government, he posthumously annulled the charges against his uncle.

[6] When Edward II came to the throne, however, he went against his father's wishes by granting the earldom of Cornwall to his favourite Piers Gaveston.

[8] Edward II, nevertheless, took steps to provide his half-brother with an income; grants made in 1315 and 1319 secured Edmund 2000 marks a year.

In the spring of 1320 he took part in an embassy to Pope John XXII in Avignon, where the mission was to absolve the king of his oath to uphold the Ordinances, a set of restrictions imposed on royal authority by the baronage.

[14] Once confronted with the royal army, Mortimer surrendered without a fight, and attention turned to the leader of the baronial opposition, Thomas of Lancaster.

Edmund was charged with overtaking Wallingford Castle from Maurice de Berkeley in January 1323, a task which he fulfilled with great success.

A major campaign was organised in August, but the effort ended in total failure when the English were routed by the Scots, led by Robert the Bruce, at the Battle of Old Byland on 14 October 1322.

Andrew Harclay, who had defeated Lancaster at Boroughbridge, and for this had been created Earl of Carlisle and appointed Warden of the Marches to Scotland, signed a peace treaty with the Scots without royal sanction in January 1323.

[22] While some historians have criticised Edmund for his failure to reach a diplomatic settlement,[23] others have pointed to the difficult circumstances he faced, and how others had fared little better.

[24] Edward II's refusal to pay homage to the French king was based on concern for his royal sovereignty, but also on fear of a potential resurgence of domestic resistance.

[26] The Queen departed for France on 9 March 1325, and in September she was joined by her son, the heir to the throne, Prince Edward.

[27] Isabella's negotiations were successful, and it was agreed that the young Prince Edward would perform homage in the king's place, which he did on 24 September.

[3] For his participation in the coup, Edmund received a reward of land belonging to the Despensers, and the Earl of Arundel, who was also executed as a supporter of Edward II.

[32] It did not take long for Edmund to grow disenchanted with the new regime; one source of contention was the dominant position at the court of Mortimer, who has been described as Isabella's lover.

[35] Upon hearing that the verdict was death, the condemned earl pleaded with Edward III for his life, offering to walk from Winchester to London with a rope around his neck as a sign of atonement, but leniency was blocked by Mortimer and the queen.

It was almost impossible to find anyone willing to perform the execution of a man of royal blood, until a convicted murderer (who was said to be responsible "for cleaning the latrines")[37] eventually beheaded Edmund in exchange for a pardon.

[39] In October 1330, Edward III carried out a coup installing himself in personal control of the government, and Mortimer was executed.

[3] At the same time, it has been pointed out that Edmund showed a great deal of loyalty to Edward II, in spite of receiving relatively little rewards and recognition from his brother.