Stephen was son of Stephen, Count of Blois, and Adela, daughter of William the Conqueror. He was born in Blois, France, in 1096. His father died in 1102, on the First Crusade, and Stephen's elder brother Thibauld succeeded to the County. In c.1115 Stephen became Count of Mortain. Stephen was 29 when he married, his wife being Matilda, daughter of the Count of Boulogne. Through his mother he had a claim on the English throne. The then-king, Henry I, however, preferred that the succession pass to his daughter, Matilda, otherwise known as Maud, who was the widow of the Holy Roman Emperor Henry V. He had his barons (including Stephen) swear to uphold her right before he died in 1135.
Stephen still fancied his own chances, and he crossed the English Channel to England, claimed the throne at Winchester (where his brother Henry was Bishop) and had himself crowned. The English baronage, aware that Matilda had remarried, to Geoffrey of Anjou, and was with her husband in France, were seduced into supporting the usurper (she was a foreigner to them, and a woman besides) and until 1139 Stephen had quiet possession of the throne.
In 1139, supported by her husband and by her brother Robert of Gloucester, Empress Matilda invaded, and by 1141 had captured King Stephen. Unfortunately she was less adept than he at keeping the sympathy of key subjects (such as the citizens of London), and Stephen escaped. In 1142 the situation was reversed, and he captured Matilda, but she escaped under her own steam.
Thereafter, until 1153, Stephen kept the throne, but in that year he suffered twin adversities. His son and heir, Eustace, died, and Matilda's son, Henry made an invasion. Faced with little prospect of maintaining his grand-father's dynasty, he agreed, at the Treaty of Wallingford, that on his death Henry should take the throne. This ensured him peace for the remainder of his reign, and he died in 1154 at Dover.
The Anglo-Saxon Chronicle is less than complementary about Stephen, assessing his reign as a time of "strife, evil and robbery", when the barons, seeing their king a "good-humoured, kindly and easy-going man", took advantage and committed many crimes. Their final judgement was that, during his 19-year reign, "men said openly that Christ and his angels slept".