Amalric was born in 1136 to King Fulk, the former count of Anjou who had married the heiress of the kingdom, Melisende, daughter of King Baldwin II. After the death of Fulk in a hunting accident in 1143, the throne passed jointly to Melisende and Amalric's older brother Baldwin III, who was still only 13 years old. Melisende did not step down when Baldwin came of age two years later, and by 1150 the two were becoming increasingly hostile towards each other. In 1152 Baldwin had himself crowned sole king, and civil war broke out, with Melisende retaining Jerusalem while Baldwin held territory further north. Amalric, who had been given the County of Jaffa as an apanage when he reached the age of majority in 1151, remained loyal to Melisende in Jerusalem, and when Baldwin invaded the south, Amalric was besieged in the Tower of David with his mother. Melisende was defeated in this struggle and Baldwin ruled alone thereafter. In 1153 Baldwin captured the Egyptian fortress of Ascalon, which was then added to Amalric's fief of Jaffa (see Battle of Ascalon).
Amalric married Agnes of Courtenay in 1157. Agnes, daughter of Joscelin II of Edessa, had lived in Jerusalem since the western regions of the former crusader County of Edessa were lost in 1150. Patriarch Fulcher objected to the marriage on grounds of consanguinity, as the two shared a great-great-grandfather, Guy I of Montlhéry, and it seems that they waited until Fulcher's death to marry. Agnes bore Amalric three children: Sibylla, the future Baldwin IV (both of whom would come to rule the kingdom in their own right), and Alix, who died in childhood.