Whittlesey was educated at Oxford, and owing principally to the fact that he was a nephew of Simon Islip, archbishop of Canterbury, he received numerous ecclesiastical preferments; he held prebends at Lichfield, Chichester and Lincoln, and livings at Ivychurch, Croydon and Cliffe.
[1] Whittlesey was briefly appointed Master of Peterhouse on 10 September 1349 and resigned from that post in 1351.
[2] Later he was appointed vicar-general, and then dean of the court of arches by Islip.
[3] Two years after his consecration he was transferred to the bishopric of Worcester on 6 March 1364.
[4] On 11 October 1368 Whittlesey was transferred to the archbishopric of Canterbury in succession to Simon Langham, but his term of office was very uneventful, a circumstance due partly, but not wholly, to his feeble health.