Cymer Abbey was founded in 1158-9 and dedicated to the Virgin Mary under the patronage of Maredudd ap Cynan ab Owain Gwynedd (d. 1212), Lord of Merioneth (grandson of Owain Gwynedd), and of his brother, Gruffudd ap Cynan, prince of N. Wales (d. 1200). It was a daughter house of Abbeycwmhir in Powys.
The Abbey was a base for the troops of Prince Llywelyn ap Gruffudd in 1275 and 1279. In 1283 Edward I occupied the Abbey and a year later gave the Abbey compensation of £80 for damage caused in the recent wars.
By 1388 the monastery was home to no more than five monks and it seems that there was a marked decline in the standard of religious observance. In the survey of 1535, the annual income of the house was valued at little over £51 and the abbey was dissolved with the smaller monasteries in 1536-7, most likely in March 1537. The monastery was small and relatively unimportant. However, Cymer did possess a fine, thirteenth century silver gilt chalice and paten (Eucharist plate), which must have been hidden at the Dissolution; rediscovered in 1898,under a stone at Cym-y-mynach, they are now in the National Museum of Wales in Cardiff.