Probably educated at Cambridge and then overseas, he held the degree of Doctor of Laws and his first career was as a canon lawyer in Avignon, which was then the seat of the Papacy.
As Fastolf advanced in the service of the Avignon popes, he gained a series of preferments in the British Isles, concluding with the bishopric of St David's.
[4] As Bishop of St David's for nine years, Fastolf appears to have presided over a period of quiet order, with no major new projects undertaken.
[5]In 1358, Fastolf made a statute, later confirmed by Richard II, to make it easier and safer for the prebendaries of his diocese to collect their share of the tithes.
As he had requested in his Will, which was proved on 1 July 1361, his body was entombed "at the right hand of the image of the Blessed Mary, in her Chapel", in St David's Cathedral.