Guy was possibly a younger son of Hugh II, Count of Ponthieu and about 1053 succeeded his brother Enguerrand II, as Count of Ponthieu. However, a contemporary charter describes Guy as Enguerrand's son. The Ponthievin alliance with Duke William of Normandy had earlier been secured by the marriage of Enguerrand to Adelaide of Normandy, Duke William's sister. But the marriage was apparently annulled due to consanguinity c.1049/50. Enguerrand's and Guy's sister was married to William of Talou, son of Richard II, Duke of Normandy. William of Talou had built a strong castle at Arques, and from it he defied his nephew the youthful Duke of Normandy. As "family", the comital house of Ponthieu supported the rebellion. Duke William put Arques under siege, and then remained mobile with another force in the countryside nearby. He was aware that Normandy was being threatened by the armies of King Henry of France, who wanted to bring his young, former vassal to heel; and that Normandy's erstwhile allies from Ponthieu would also be coming to break the siege of Arques. Young Count Enguerrand led a Ponthievin army into the Talou to relieve Arques, and arrived first, but Duke William successfully ambushed them and Enguerrand was killed. Upon learning of this serious reverse, the vacillating Henry withdrew his forces at once back across the Norman border. William of Talou was compelled to surrender Arques and was banished for life. With the death of his older brother, Guy assumed the comital duties: this is the first mention of Guy in the historical record. In February 1054, Henry was again ready to chastise Duke William: he reentered the duchy with a large army of his own liegemen and an Angevin army led by Count Geoffrey of Anjou. This combined force moved down the Seine toward Rouen, while Henry's brother Eudes "led" a second army, along with Guy and Count Rainald of Clairmont. The Franco-Ponthievin army was undisciplined, and fragmented out of control to plunder and pillage the countryside around Mortemer. They were attacked suddenly by Normans from Eu and other districts of northeastern Normandy. In the Battle of Mortemer, Guy's younger brother Waleran was mortally wounded, and Guy himself was captured. He spent two years as a prisoner in Normandy, while his uncle, Bishop Guy of Amiens, ruled Ponthieu as regent. Evidently, from this point on, Count Guy was a vassal of Duke William of Normandy.
In 1064, Harold Godwinson, the Earl of Wessex, was shipwrecked on the shores of Ponthieu and captured by Count Guy who took him to his castle of Beaurain on the River Canche, as the Bayeux Tapestry relates: hic apprehendit wido Haroldum et duxit em ad Belrem et ibi eum tenuit. Duke William demanded the release of the earl, and Count Guy delivered Harold Godwinson up after being paid a ransom for him. Harold was not released from Normandy until he too had sworn on the Holy Relics to be Duke William's vassal, and to aid him to the throne of England. In 1066, Harold accepted the crown of England upon the death of Edward the Confessor, thus precipitating the war that resulted in the Norman Conquest. According to a very convincing interpretation of The Carmen de Hastingae Proelio, Hugh, another of Guy's brothers, was a participant in the Battle of Hastings, and had a hand in the slaying of Harold. Guy I had a son, Enguerrand, who must have died before the Carmen was composed : when the Carmen refers to Hugh, Guy's brother, as "the noble heir of Ponthieu", we must assume Enguerrand's death as a fact, either at the time of the Conquest, or shortly before.
