In the dispute between his mother, and his older brother Baldwin III, over the governance of the Kingdom of Jerusalem, Amalric had sided with Melisende, and in 1152 Baldwin besieged them both in the citadel of Jerusalem, the Tower of David. However, when Baldwin obtained what he had sought, the undisputed crown, he did not hold Amalric's filial loyalty against him and when, a year later, in 1153, he captured Ascalon, he added it to Amalric's fief of Jaffa.
In 1157 Amalric married the daughter of the Count of Edessa, although the marriage may have been delayed to allow a patriarch, who objected to it on the grounds of consanguinity, to pass away. Amalric and Agnes had 2 children, a daughter Sybilla and a son Baldwin who were both, in due course, to rule over the Kingdom of Jerusalem.
Baldwin III died in 1162, childless -- Amalric was the heir, but the consanguinity argument reared his head again and, in order to succeed, he was obliged to have his marriage annulled (although the Church ruled their children legitimate). He allowed Agnes to remain Countess of Jaffa and ascended the throne wifeless. Agnes then remarried, to Hugh of Ibelin, to whom she had been affianced prior to marrying Amalric.
Alamric was obliged to go to war almost at once, against the Muslims of Egypt, which took up the first 4 years of his reign. Returning to Jerusalem in 1167, he then married Maria Comnena, grand-daughter to the Byzantine Emperor Manuel I. King and emperor then negotiated an alliance against Egypt, and a brief campaign appeared to have been successful. In truth, however, the war merely made matters worse for Amalric and his people, since it brought to power one of the mightiest Sultans of Egypt, the former Vizier, Yusuf Salah al-Din, better known to the West as Saladin.
In 1170 Saladin invaded Jerusalem and seized Eilat; thereafter he and the Syrian Muslims were a constant threat. In 1174, perceiving a weakness among his enemies, Amalric attacked the city of Banias. He contracted dysentery, which turned to fever, and in July 1174 he died, leaving a 2-year-old daughter by Maria, Isabella, who was also destined to succeed as Queen. He was succeeded by his son, Baldwin IV, also known as "Baldwin the Leper", for he suffered that affliction.