In 1257, Edmund joined his parents on their first visit to Germany, to pursue Richard's nominal or claimant title as king to the Holy Roman Empire, returning in January 1259.

Following Richard's death on 2 April 1272, Edmund was recognised as his heir, and swore homage to the king for his father's estates on or before 1 May 1272.

On 13 October, on the feast of Edward the Confessor, Edmund was knighted by Henry III at Westminster Abbey and invested with his father's honours and titles as Earl of Cornwall.

When Henry III died in November 1272, Edmund took a post in the governing council in England, and was among the councillors who wrote to Edward I to advise him of his father's death.

In June 1273 he travelled to France to meet Edward I, and two months later, in Paris, acknowledged the repayment of 2,000 of the 3,000 marks that the king owed him.

Between April 1282 and December 1284 Edmund served as Edward I's lieutenant in the government whilst the king conducted a campaign in Wales, mediated the collection of the clerical subsidy towards the costs of the proposed crusade, ensured the exchequer rolls were transported to Shrewsbury, attended a clerical convocation in Northampton in January 1283 as the king's representative, as well as taking custody of wardships and estates on his personal account.

Edward's return to England was followed by an inquiry into wrongdoings by the government during his absence, and though several judges and officials were disgraced, Edmund was retrospectively pardoned for any forest offences and allowed to answer by proxy any complaints against his administration in Cornwall, where he was High Sheriff from 1289 to 1300.

Edward I celebrated Christmas of that year at Edmund's manor at Ashridge in Hertfordshire, where he held parliament and discussed the business of Scotland.

In July 1297 Edmund was granted licence to make a will, his poor health is mentioned in a summons of December 1298, and by 1300 he was terminally ill.