Samson was of noble blood, the son of Amon of Dyfed and Anna of Gwent. His father's brother also married his mother's sister, and their son, Saint Magloire, was his cousin. As part of the prophecy concerning his birth, his parents placed him under the care of Saint Iltud, abbot of Llantwit to become a monk. Samson later became abbot of Llantwit. He was ordained bishop by bishop Dubricius on the Feast of the Chair of St Peter (February 22) at the beginning of Lent -- which can be calculated to have fallen in the year 521, the only certain date in Samson's life. If the usual practice was observed and he was 35 years old at the time of his ordination, this would mean he was born in 486; however, he is recorded as having been in attendance at a church Council in Paris held at some time between 556 and 573, had been born in 486, he would have been at least 70 years old at the time of this Council.
Samson was abbot at several different monasteries before he was ordained bishop, and following his ordination, he travelled to Ireland, South Wales, Cornwall and Brittany. In Cornwall he founded a monastery that was located at either South Hill or Golant, and in Brittany he founded the monastery of Dol. Samson also participated in Breton politics, successfully petitioning the Merovingian king Childebert I on behalf of king Juduwal. He was buried, with his cousin Magloire, in the Cathedral of Dol.
The primary source for his biography is the Vita Sacti Samsonis, written sometime between 610 and 820, but clearly based on earlier materials. Not only does it preserve such details about Samson such as his abstinence from alcohol -- unlike many of his contemporaries, such as the abbot Piro who was killed when he fell down a well while drunk -- but valuable details about Celtic Christianity in Britain during Samson's time. This document details the contacts churchmen in Britain had with both Ireland and Brittany, details of their belief, as well as offering facts that have been used to prove both that religious communities were headed by abbots where the bishops served in a subordinate role, and that these communities were actually headed by bishops as was the usual practice in the rest of Europe. This Vita was later used as a model for the writing of other saint lives in Brittany.