Cressen was the maester at Storm's End for many years and a loyal servant of House Baratheon. He gave loyal service to Lord Robert Baratheon and, after his elevation to the Iron Throne, his brothers Stannis and Renly. After Stannis was appointed Lord of Dragonstone, Cressen continued to serve him there.
Before and during the early part of the War of the Five Kings Cressen became disturbed by the arrival of Melisandre of Asshai and the influence she exerted over Stannis and his wife. He eventually concluded that she was a danger to the family and had to be eliminated. His attempt to poison her failed as the poison strangely did not affect her but did affect him, resulting in his death. He was replaced by Pylos.
Pate, the POV character in the prologue of A Feast for Crows mentions that Archmaester Walgrave, keeper of the ravens at the Citadel, sometimes mistakes Pate for someone named Cressen.