He was the younger son of Geoffrey, Count of Gâtinais (sometimes known as Aubri), and Ermengarde of Anjou, a daughter of Fulk the Black, count of Anjou, and sister of Geoffrey Martel, also count of Anjou.
When Geoffrey Martel died without direct heirs he left Anjou to his nephew Geoffrey III of Anjou, Fulk le Réchin's older brother.
Fulk fought with his brother, whose rule was deemed incompetent, and captured him in 1067. Under pressure from the Church he released Geoffrey. The two brothers soon fell to fighting again, and the next year Geoffrey was again imprisoned by Fulk, this time for good.
Substantial territory was lost to Angevin control due to the difficulties resulting from Geoffrey's poor rule and the subsequent civil war. Saintonge was lost, and Fulk had to give the Gâtinais to Philip I of France to placate the king.
In 1096 Fulk wrote an incomplete history of Anjou and its rulers, though the authorship and authenticity of this work is disputed. If he did write it, it is one of the first medieval works of history written by a layman.
Fulk may have married as many as five times; there is some doubt regarding two of the marriages.
His first wife was Hildegarde of Baugency. After her death, before 1070, he married Ermengarde de Borbon, and then possibly Orengarde de Châtellailon. Both these were repudiated (Ermengarde de Borbon in 1075 and Orengarde de Chatellailon in 1080), possibly on grounds of consanguinity.
Next he married Bertrade de Montfort, who apparently left him for Philip I of France. Finally, he may have married Mantie, daughter of Walter I of Brienne. This marriage also ended in divorce, in 1087.
He had two sons. The eldest (a son of Ermengarde de Borbon), Geoffrey Martel II, Geoffrey IV of Anjou, ruled jointly with his father for some time, but died in 1106. The younger (a son of Bertrade de Montfort) succeeded as Fulk V.