He had obtained a licentiate in decrees at some point in his youth, and he must have become a Tironensian monk, for by 1329, and perhaps as early as 1327, he was Abbot of Kilwinning.
In 1344, Deyn, perhaps through Stewart patronage, was chosen to succeed Alexander de Kininmund as Bishop of Aberdeen.
In May 1345, the Pope appointed him as Scotland's papal tax collector, a duty Deyn fulfilled, sending the proceeds to the papacy via merchants in Bruges.
He was one of the notables who petitioned the papacy in 1347 to legitimise the marriage of Robert Stewart and Elizabeth More of Rowallan.
In 1349 he was in attendance with the Justiciar of Scotia holding court at the standing stones of Old Rayne in Garioch.