Clyn was probably born in Leinster some years prior to 1300, possibly at Baile a Clinn/Clyn's town/Clintstown, in the parish of Conahy, some six and a half miles north-west of Kilkenny.
Bishop James Ussher stated, possibly in error, that Clyn was a doctor of the Franciscan Order.
Clyn is recorded as the Guardian of the friary of Carrick in 1336; Bernadette Williams believes that he would have been about fifty at that time ("around the same age as his hero Fulk de la Freigne"), as someone mature would have been needed to hold that post.
John Clyn wrote a diary, or journal, about the Black Death, which has become a key primary source on the plague.
... the difference between a city and county annalist is quite evident; Clyn was not a member of the burgage population of Kilkenny but a man of the countryside.