1332 for a brief tenure of the archdeaconry of Stow (1332–3), and the last he vacated by cession in 1357 (Le Neve, Fasti Eccl. Anglic. ii. 78, ed. Hardy). He held the prebend of Welton Brinkhall, in the cathedral of Lincoln, from 1327 till 1331 (ib. ii. 228). In 1329 he was collated to the prebend of Aylesbury in the same cathedral, which he exchanged in 1340 for that of Welton Beckhall (ib. ii. 96, but cf. ii. 225). In 1337 he was vicar-general to the Bishop of Lincoln. In 1343 he was made archdeacon of Canterbury, but in 1346 he surrendered that post to Peter Rogier, afterwards Pope Gregory XI (ib. i. 40). He also became dean of arches, and in 1348 prebendary of Mora in St. Paul's Cathedral on the presentation of the king (ib. ii. 410). In March 1348 he was also collated to the prebend of Sandiacre in Lichfield (ib. i. 624).
Islip attached himself to the king's service, becoming in turn chaplain, secretary, councillor, and keeper of the privy seal to Edward III. On 4 Jan. 1342 he was one of the ambassadors sent to treat for a truce with France at Antoing, near Tournay, on 3 Feb. (Fœdera, ii. 1185, Record ed.). On 1 July 1345 he was appointed, with other members of the council, to assist the king's son Lionel, while acting as regent during the king's absence abroad (ib. iii. 50). In 1346 he was authorised to open royal letters and treat with foreign ambassadors during Edward III's residence beyond sea (ib. iii. 85).
Archbishop Stratford had died on 23 Aug. 1348. His successor, John Ufford, died of the Black Death on 20 May 1349, before he was consecrated. On 26 Aug. the famous scholastic Bradwardine [q. v.] died of the same pestilence, only a week after he had received the temporalities of the see. On 20 Sept. the monks of Christ Church elected Islip, at the king's request, to the vacant archbishopric (Wharton, Anglia Sacra, i. 119); but on 7 Oct. Pope Clement VI, also in obedience to a royal request, conferred the primacy upon him by provision (ib. i. 376). On 20 Dec. 1349 Islip was consecrated at St. Paul's. He received the pallium on 25 March 1350 at Esher from Bishop Edington. As the Black Death had not yet ceased its ravages, he caused himself to be enthroned privately at Canterbury (ib. i. 377), and without the usual lavish festivities. The Christ Church monks, who already resented his consecration out of Canterbury, unfairly attributed the absence of the customary entertainments to his parsimony, and a reputation for niggardliness remained to him for the rest of his life. On 23 April 1350 Islip assisted at the gorgeous pageant at Windsor in which Edward III inaugurated the order of the Garter (G. Le Baker, pp. 109, 278–9, ed. Thompson). He long remained very poor, and he incurred much reproach for cutting down and selling the timber on his estates; for exacting larger sums from his clergy than he had received papal authority to exact; for dealing hardly with the executors of Ufford in the matter of dilapidations; and for alienating for ready money the perpetual right of the archbishops to receive from the Earls of Arundel a yearly grant of twenty-six deer.
Islip's diocese had been demoralised by the ravages of the Black Death, and in an early visitation he sought energetically to remedy the evils. He afterwards visited ‘perfunctorily’ the dioceses of Rochester and Chichester, but subsequently remained mostly in his manors, of which Mayfield in Sussex soon became his favourite residence. In 1356 he was specially exhorted by Innocent VI to resume his visitations (Wilkins, Concilia, iii. 35–6). Islip was never lacking in vigilance, and strove earnestly to restore discipline (cf. his constitutions and canons in Wilkins, vol. iii.). He deprived criminous clerks of their benefices; took care that clerks incarcerated in ecclesiastical prisons should not fare too well; and enforced a stricter keeping of Sunday, especially by putting down markets and riotous gatherings on that day. He directed, however, that work should not be suspended on minor saints' days (Walsingham, Hist. Angl. i. 297, Rolls Ser.) The plague had thinned the ranks of the beneficed clergy, and unbeneficed priests now refused to undertake pastoral work for the stipends customary before the Black Death. Many parishes were thus wholly or in part deprived of spiritual direction. Islip therefore issued in 1350 a canon which is a sort of spiritual counterpart of the Statute of Labourers, ordering chaplains to remain content with the salaries they had received before the Black Death (Wilkins, iii. 1–2). In 1362, the year after the second visitation of the Black Death had intensified existing evils, Islip drew up other constitutions defining more strictly the priests' remuneration, and ordering the deprivation of those who refused to undertake pastoral functions when called upon by the bishop (ib. iii. 50). Islip's measures drove many priests to theft (Walsingham, i. 297). In 1353 Islip also drew up regulations for the apparel and salaries of priests (Wilkins, iii. 29). His care for the secular clergy led him to limit the rights of the friars to hear confessions or discharge pastoral functions (ib. iii. 64).
In 1353 Islip arranged with Archbishop