Following the final defeat of the Lancastrian cause at the
Battle of Tewkesbury on 4 May 1471, Morton was granted a pardon by Edward IV and resumed his career in royal service. By 29 September 1471 (Michaelmas) he was appointed as a Master in Chancery and on 16 March 1472 he was granted the office of
Master of the Rolls. Morton began to accumulate more benefices, being made Rector of St Dunstan-in-the-East in London in 1472 (exchanged in 1474 for appointments as Rector of South Molton in Devon and master of St Bartholomew's Hospital in Bristol), a prebend of St Paul's in London in 1473, archdeacon of Chester (1474), Winchester (1475), Huntingdon (1475), Berkshire (1476), Norfolk (1477) and Leicester (1478) and a prebend of Wells (1476), York (1476) and Exeter (1476). Civil lawyers were also in demand for diplomatic missions and Morton left England in early January 1474 together with Lord Duras on an embassy to Burgundy. He did not return until early June. In December 1474 he was despatched on another embassy to Burgundy along with Sir Thomas Montgomery and the king's secretary, William Hatclyf. They were also commissioned to seek alliances against France with the Holy Roman Emperor, Frederick III, and Matthias Corvinus, King of Hungary. After Edward IV invaded France in the middle of 1475, Morton was involved in negotiating the
Treaty of Picquigny (29 August 1475) by which
Louis XI undertook to pay Edward IV 75,000 crowns within fifteen days, and 50,000 crowns yearly thereafter as long as they both lived, in exchange for the withdrawal of the English army. Along with other members of the English court, Morton was rewarded by Louis XI with an annual pension, in his case of 600 crowns. On 16 February 1477 Edward IV sent Morton and
Sir John Donne as ambassadors to Louis XI to seek the extension of the truce under the Treaty of Picquigny. When Louis XI sent ambassadors to England in July 1477 to continue negotiations, Morton was one of those appointed to meet them. By a treaty made on 21 July 1477 the truce was extended for the lives of the kings and for a year after the death of whichever of them died first. On 21 January 1479 Edward IV instructed Morton and others to begin negotiations with the French ambassador, Charles de Martigny, Bishop of Elne, with a view to extending the truce between the two nations to last for one hundred and one years and to pay the pension of 50,000 crowns yearly throughout that period to Edward and his successors. In February 1479 the bishop signed agreements to that effect drawn up by Morton. In the meanwhile, Morton had been consecrated by Archbishop Bourchier as Bishop of Ely in the chapel at Lambeth on 31 January 1479 and he vacated the other ecclesiastical offices which he had accumulated. It was during this period that Morton was mentioned by the visiting Italian observer, Dominic Mancini, as being, along with
Archbishop Thomas Rotherham and
William, Lord Hastings, "of no small influence" with the king. Mancini wrote that "these men being in age mature, and instructed by long experience of public affairs, helped more than other councillors to form the king's policy, and besides carried it out". As Bishop of Ely, Morton initiated various building works, including rebuilding the Bishop's Palace at Hatfield in Hertfordshire which had been in the possession of the Bishops of Ely since 1109. Morton rebuilt it in about 1480 as a stately manor house, all in brick. He was also responsible for the construction of Morton's Leam, a cut or ditch twelve miles in length, forty feet broad and four feet deep, from
Stanground near Peterborough to
Guyhirn, by which water was channelled from the
River Nene to
Wisbech. Morton had a tower built to monitor the works. == Exile under Richard III ==